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Institute
Sonstige beteiligte Institutionen
- LMU München (1)
- Universität Bielefeld (1)
Investigation of Nanostructure-Induced Localized Light Phenomena Using Ultrafast Laser Spectroscopy
(2017)
In recent years, the interaction of light with subwavelength structures, i.e., structures that are smaller than the optical wavelength, became more and more interesting to scientific research, since it provides the opportunity to manipulate light-induced dynamics below the optical diffraction limit. Specifically designed nanomaterials can be utilized to tailor the temporal evolution of electromagnetic fields at the nanoscale. For the investigation of strongly localized processes, it is essential to resolve both their spatial and their temporal behavior. The aim of this thesis was to study and/or control the temporal evolution of three nanostructure-induced localized light phenomena by using ultrafast laser spectroscopy with high spatial resolution.
In Chapter 4, the absorption of near-infrared light in thin-film a-Si:H solar cells was investigated. Using nanotextured instead of smooth interfaces for such devices leads to an increase of absorption from < 20% to more than 50% in the near-infrared regime. Time-resolved experiments with femtosecond laser pulses were performed to clarify the reason for this enhancement. The coherent backscattered radiation from nanotextured solar cell devices was measured as a function of the sample position and evaluated via spectral interferometry. Spatially varying resonance peaks in the recorded spectra indicated the formation of localized photonic modes within the nanotextured absorber layers. In order to identify the modes separately from each other, coherent two-dimensional (2D) nanoscopy was utilized, providing a high spatial resolution < 40 nm. In a nanoscopy measurement on a modified device with an exposed nanotextured a-Si:H absorber layer, hot-spot electron emission was observed and confirmed the presence of localized modes. Fitting the local 2D nanospectra at the hot-spot positions enabled the determination of the resonance frequencies and coherence lifetimes of the modes. The obtained lifetime values varied between 50 fs and 130 fs. Using a thermionic emission model allowed the calculation of the locally absorbed energy density and, with this, an estimation of the localization length of the photonic modes (≈1 μm). The localization could be classified by means of the estimated localization length and additional data evaluation of the backscattered spectra as strong localization ─ the so-called Anderson localization.
Based on the experimental results, it was concluded that the enhanced absorption of near-infrared light in thin-film silicon solar cells with nanotextured interfaces is caused by the formation of strongly localized photonic modes within the disordered absorber layers. The incoming near-infrared light is trapped in these long-living modes until absorption occurs.
In Chapter 5, a novel hybridized plasmonic device was introduced and investigated in both theory and experiment. It consists of two widely separated whispering gallery mode (WGM) nanoantennas located in an elliptical plasmonic cavity. The goal was to realize a periodic long-range energy transfer between the nanoantennas. In finite-difference time-domain (FDTD) simulations, the device was first optimized with respect to strong coupling between the localized antenna modes and the spatially-extended cavity mode. The geometrical parameters of the antennas and the cavity were adjusted separately so that the m="0" antenna mode and the cavity mode were resonant at λ="800 nm" . A high spatial overlap of the modes was achieved by positioning the two antennas in the focal spots of the cavity, leading to a distance between the antenna centers of more than twice the resonant wavelength of the modes. The spectral response of the optimized device revealed an energy splitting of the antenna and the cavity mode into three separated hybridized eigenmodes within an energy range of about 90 meV due to strong coupling. It could be well reproduced by a simple model of three coupled Lorentzian oscillators. In the time domain, an oscillatory energy transfer between both antennas with a period of 86 fs and an energy transfer efficiency of about 7% was observed for single-pulse excitation. For the experiments, devices with cavities and antennas of varying size were fabricated by means of focused-ion-beam (FIB) milling. Time-resolved correlation measurements were performed with high spatial and temporal resolution by using sequences of two femtosecond laser pulses for excitation and photoemission electron microscopy (PEEM) for detection. Local correlation traces at antennas in resonant devices, i.e., devices with enhanced electron emission at both antenna positions, were investigated and reconstructed by means of the coupled-oscillator model. The corresponding spectral response revealed separated peaks, confirming the formation of hybridized eigenmodes due to strong coupling. In a subsequent simulation for single-pulse excitation, one back-and-forth energy transfer between both antennas with an energy transfer efficiency of about 10% was observed.
Based on the theoretical and experimental results, it was demonstrated that in the presented plasmonic device a periodic long-range energy transfer between the two nanoantennas is possible. Furthermore, the coupled-oscillator model enables one to study in depth how specific device properties impact the temporal electric-field dynamics within the device. This can be exploited to further optimize energy transfer efficiency of the device. Future applications are envisioned in ultrafast plasmonic nanocircuitry. Moreover, the presented device can be employed to realize efficient SPP-mediated strong coupling between widely separated quantum emitters.
In Chapter 6, it was investigated in theory how the local optical chirality enhancement in the near field of plasmonic nanostructures can be optimized by tuning the far-field polarization of the incident light. An analytic expression was derived that enables the calculation of the optimal far-field polarizations, i.e., the two far-field polarizations which lead to the highest positive and negative local optical chirality, for any given nanostructure geometry. The two optimal far-field polarizations depend on the local optical response of the respective nanostructure and thus are functions of both the frequency ω and the position r. Their ellipticities differ only in their sign, i.e., in their direction of rotation in the time domain, and the angle between their orientations, i.e., the angle between the principal axes of their ellipses, is ±π/"2" . The handedness of optimal local optical chirality can be switched by switching between the optimal far-field polarizations. In numerical simulations, it was exemplarily shown for two specific nanostructure assemblies that the optimal local optical chirality can significantly exceed the optical chirality values of circularly polarized light in free space ─ the highest possible values in free space. The corresponding optimal far-field polarizations were different from linear and circular and varied with frequency. Using femtosecond polarization pulse shaping provides the opportunity to coherently control local optical chirality over a continuous frequency range. Furthermore, symmetry properties of nanostructures can be exploited to determine which far-field polarization is optimal.
The theoretical findings can have impact on future experimental studies about local optical chirality enhancement. Tuning the far-field polarization of the incident light offers a promising tool to enhance chirally specific interactions of local electromagnetic fields with molecular and other quantum systems in the vicinity of plasmonic nanostructures. The presented approach can be utilized for applications in chiral sensing of adsorbed molecules, time-resolved chirality-sensitive spectroscopy, and chiral quantum control.
In conclusion, each of the localized light phenomena that were investigated in this thesis ─ the enhanced local absorption of near-infrared light due to the formation of localized photonic modes, the periodic long-range energy transfer between two nanoantennas within an elliptical plasmonic cavity, and the optimization of local optical chirality enhancement by tuning the far-field polarization of the incident light ─ can open up new perspectives for a variety of future applications.
.
Der Schwerpunkt dieser Arbeit lag auf der spektroelektrochemischen Untersuchung von halbleitenden SWNTs. Hierbei wurden erstmalig Absorptions- und Photolumineszenzspektren ein und derselben SWNT-Probe simultan unter elektrochemischer Potentialkontrolle aufgenommen. Hierbei konnte gezeigt werden, dass die Messmethode einen entscheidenden Einfluss auf die erhaltene Bandlücke besitzt und der in der Literatur geprägte Begriff der Elektrochemischen Bandlücke aufgrund einer fehlenden allgemeingültigen Definition problembehaftet ist. So ergeben Photolumineszenzmessungen im Vergleich zu Raman- oder Absorptionsmessungen die kleinste Bandlücke. Dies wurde auf die diffusionskontrollierte Löschung der Exzitonen an Ladungszentren zurückgeführt. Weiterhin wurden die optischen Spektren von SWNTs unter Ladungseinfluss analysiert und die zugrundeliegenden Änderungen der elektronischen Eigenschaften diskutiert.
Neben SWNTs wurden die Übergangsmetalldichalkogenide MoS2 und WS2 spektroelektrochemisch untersucht. Auffallend im Vergleich zu den Messungen an SWNTs war der breite Potentialbereich, über den die Abnahme der exzitonischen Signale zu beobachten war. Dies kann auf die unterschiedliche elektronische Struktur von TMDs und SWNTs und den geringen Anteil von Einzellagen in den TMD-Proben zurückgeführt werden. Weiterhin konnte in den Absorptionsspektren unter Ladungseinfluss ein Signal beobachtet werden, welches auf die Entstehung von Trionen hindeutet.
In einem weiteren Teilprojekt wurde eine elektrochemische Zelle zur Untersuchung von metallischen SWNT-Filmen als Elektrode für die Wasserstoffproduktion entwickelt und getestet. Hierbei gelang es die von Das et al. publizierte Aktivierung von SWNTs mit Schwefelsäure erfolgreich nachzuvollziehen und einen katalytischen Effekt der SWNTs auf die Wasserstoffentwicklung zu beobachten.
This thesis will outline studies performed on the fluorescence dynamics of phenyl-benzo-
[c]-tetrazolo-cinnolium chloride (PTC) in alcoholic solutions with varying viscosity using
time-resolved fluoro-spectroscopic methods. Furthermore, the properties of femtosecond
Laguerre-Gaussian (LG) laser pulses will be investigated with respect to their temporal
and spatial features and an approach will be developed to measure and control the spatial
intensity distribution on the time scale of the pulse.
Tetrazolium salts are widely used in biological assays for their low oxidation and reduction
thresholds and spectroscopic properties. However, a neglected feature in these applications
is the advantage that detection of emitted light has over the determination of the
absorbance. To corroborate this, PTC as one of the few known fluorescent tetrazolium
salts was investigated with regard to its luminescent features. Steady-state spectroscopy
revealed how PTC can be formed by a photoreaction from 2,3,5-triphenyl-tetrazolium
chloride (TTC) and how the fluorescence quantum yield behaved in alcoholic solvents
with different viscosity. In the same array of solvents time correlated single photon counting
(TCSPC) measurements were performed and the fluorescence decay was investigated.
Global analysis of the results revealed different dynamics in the different solvents, but
although the main emission constant did change with the solvent, taking the fluorescence
quantum yield into consideration resulted in an independence of the radiative rate from
the solvent. The non-radiative rate, however, was highly solvent dependent and responsible
for the observed solvent-related changes in the fluorescence dynamics. Further studies
with the increased time resolution of femtosecond fluorescence upconversion revealed an
independence of the main emission constant from the excitation energy, however the dynamics
of the cooling processes prior to emission were prolonged for higher excitation
energy. This led to a conceivable photoreaction scheme with one emissive state with a
competing non-radiative relaxation channel, that may involve an intermediate state.
LG laser beams and their properties have seen a lot of scientific attention over the past two
decades. Also in the context of new techniques pushing the limit of technology further to
explore new phenomena, it is essential to understand the features of this beam class and
check the consistency of the findings with theoretical knowledge. The mode conversion
of a Hermite-Gaussian (HG) mode into a LG mode with the help of a spiral phase plate
(SPP) was investigated with respect to its space-time characteristics. It was found that
femtosecond LG and HG pulses of a given temporal duration share the same spectrum
and can be characterized using the same well-established methods. The mode conversion
proved to only produce the desired LG mode with its characteristic orbital angular momentum
(OAM), that is conserved after frequency doubling the pulse. Furthermore, it
was demonstrated that temporal shaping of the HG pulse does not alter the result of its
mode-conversion, as three completely different temporal pulse shapes produced the same
LG mode. Further attention was given to the sum frequency generation of fs LG beams
and dynamics of the interference of a HG and a LG pulse. It was found that if both are
chirped with inverse signs the spatial intensity distribution does rotate around the beam
axis on the time scale of the pulse. A strategy was found that would enable a measurement
of these dynamics by upconversion of the interference with a third gate pulse. The results
of which are discussed theoretically and an approach of an experimental realization had
been made. The simulated findings had only been reproduced to a limited extend due to
experimental limitations, especially the interferometric stability of the setup.
An experimental setup for probing ultrafast dynamics at the diffraction limit was developed, characterized and demonstrated in the scope of the thesis, aiming for optical investigations while simultaneously approaching the physical limits on the length and timescale.
An overview of this experimental setup was given in Chapter 2, as well as the considerations that led to the selection of the individual components. Broadband laser pulses with a length of 9.3 fs, close to the transform limit of 7.6 fs, were focused in a NA = 1.4 immersion oil objective, to the diffraction limit of below 300 nm (FWHM).
The spatial focus shape was characterized with off-resonance gold nanorod scatterers scanned through the focal volume. For further insights into the functionality and limitations of the pulse shaper, its calibration procedure was reviewed. The deviations between designed and experimental pulse shapes were attributed to pulse-shaper artifacts, including voltage-dependent inter-layer as well as intra-layer LCD-pixel crosstalk, Fabry-Pérot-type reflections in the LCD layers, and space-time coupling. A pixel-dependent correction was experimentally carried out, which can be seen as an extension of the initial calibration to all possible voltage combinations of the two LCD layers.
The capabilities of the experimental setup were demonstrated in two types of experiments, targeting the nonlinearity of gold (Chapter 3) as well as two-dimensional spectroscopy at micro-structured surfaces (Chapter 4).
Investigating thin films, an upper bound for the absolute value for the imaginary part of the nonlinear refractive index of gold could be set to |n′′ 2 (Au)| < 0.6·10−16 m2/W, together with |n′ 2 (Au)| < 1.2·10−16 m2/W as an upper bound for the absolute value of the real part. Finite-difference time-domain simulations on y-shaped gold nanostructures indicated that a phase change of ∆Φ ≥ 0.07 rad between two plasmonic modes would induce a sufficient change in the spatial contrast of emission to the far-field to be visible in the experiment. As the latter could not be observed, this value of ∆Φ was determined as the upper bound for the experimentally induced phase change. An upper bound of 52 GW/cm2 was found for the damage threshold.
In Chapter 4, a novel method for nonlinear spectroscopy on surfaces was presented. Termed coherent two-dimensional fluorescence micro-spectroscopy, it is capable of exploring ultrafast dynamics in nanostructures and molecular systems at the diffraction limit. Two-dimensional spectra of spatially isolated hotspots in structured thin films of fluorinated zinc phthalocyanine (F16ZnPc) dye were taken with a 27-step phase-cycling scheme. Observed artifacts in the 2D maps were identified as a consequence from deviations between the desired and the experimental pulse shapes. The optimization procedures described in Chapter 2 successfully suppressed the deviations to a level where the separation from the nonlinear sample response was feasible.
The experimental setup and methods developed and presented in the scope of this thesis demonstrate its flexibility and capability to study microscopic systems on surfaces. The systems exemplarily shown are consisting of metal-organic dyes and metallic nanostructures, represent samples currently under research in the growing fields of organic semiconductors and plasmonics.
The controlled shaping of ultrashort laser pulses is a powerful technology and applied in many laser laboratories today. Most of the used pulse shapers are only able to produce linearly polarized pulses shaped in amplitude and phase. Some devices are also capable of producing limited time-varying polarization profiles, but they are not able to control the amplitude. However, for some state-of-the-art non-linear time-resolved methods, such as polarization-enhanced two-dimensional spectroscopy, the possibility of controlling the amplitude and the polarization simultaneously is desirable.
Over the last years, different concepts have been developed to overcome these restrictions and to manipulate the complete vector-field of an ultrashort laser pulse with independent control over all four degrees of freedom - phase, amplitude, orientation, and ellipticity. The aim of this work was to build such a vector-field shaper. While the basic concept used for our setup is based on previous designs reported in the literature, the goal was to develop an optimized optical design that minimizes artifacts, allowing for the generation of predefined polarization pulse sequences with the highest achievable accuracy.
In Chapter 3, different approaches reported in the literature for extended and unrestricted vector-field control were examined and compared in detail. Based on this analysis, we decided to follow the approach of modulating the spectral phase and amplitude of two perpendicularly polarized pulses independently from each other in two arms of an interferometer and recombining them to a single laser pulse to gain control over the complete vector field.
As described in Chapter 4, the setup consists of three functional groups: i) an optical component to generate and recombine the two polarized beams, ii) a 4f setup, and iii) a refracting telescope to direct the two beams under two different angles of incidence onto the grating of the 4f setup in a common-path geometry. This geometry was chosen to overcome potential phase instabilities of an interferometric vector-field shaper. Manipulating the two perpendicularly polarized pulses simultaneously within one 4f setup and using adjacent pixel groups of the same liquid-crystal spatial light modulator (LC SLM) for the two polarizations has the advantages that only a single dual-layer LC SLM is required and that a robust and compact setup was achieved. The shaping capabilities of the presented design were optimized by finding the best parameters for the setup through numerical calculations to adjust the frequency distributions for a broad spectrum of 740 – 880 nm. Instead of using a Wollaston prism as in previous designs, a thin-film polarizer (TFP) is utilized to generate and recombine the two orthogonally polarized beams. Artifacts such as angular dispersion and phase distortions along the beam profile which arise when a Wollaston prism is used were discussed. Furthermore, it was shown by ray-tracing simulations that in combination with a telescope and the 4f setup, a significant deformation of the beam profile would be present when using a Wollaston prism since a separation of the incoming and outgoing beam in height is needed. The ray-tracing simulations also showed that most optical aberrations of the setup are canceled out when the incoming and outgoing beams propagate in the exact same plane by inverting the beam paths. This was realized by employing a TFP in the so-called crossed-polarizer arrangement which has also the advantage that the polarization-dependent efficiencies of the TFP and the other optics are automatically compensated and that a high extinction ratio in the order of 15000:1 is reached. Chromatic aberrations are, however, not compensated by the crossed-polarizer arrangement. The ray-tracing simulations confirmed that these chromatic aberrations are mainly caused by the telescope and not by the cylindrical lens of the 4f setup. Nevertheless, in the experimentally used wavelength range of 780 – 816 nm, only minor distortions of the beam profile were observed, which were thus considered to be negligible in the presented setup.
The software implementation of the pulse shaper was reviewed in Chapter 5 of this thesis. In order to perform various experiments, five different parameterizations, accounting for the extended shaping capabilities of a vector-field shaper, were developed. The Pixel Basis, the Spectral Basis, and the Spectral Taylor Basis can generally be used in combination with an optimization algorithm and are therefore well suited for quantum control experiments. For multidimensional spectroscopy, the Polarized Four-Pulse Basis was established. With this parameterization pulse sequences with up to four subpulses can be created. The polarization state of each subpulse can be specified and the relative intensity, phase, and temporal delay between consecutive subpulses can be controlled. In addition, different software programs were introduced in Chapter 5 which are required to perform the experiments conducted in this work.
The experimental results were presented in Chapter 6. The frequency distribution across the LC SLM was measured proving that the optimal frequency distribution was realized experimentally. Furthermore, the excellent performance of the TFP was verified. In general, satellite pulses are emitted from the TFP due to multiple internal reflections. Various measurements demonstrated that these pulses are temporally separated by at least 4.05 ps from the main pulse and that they have vanishing intensity. The phase stability between the two arms of the presented common-path setup σ = 28.3 mrad (λ/222) over 60 minutes. To further improve this stability over very long measurement times, an on-the-fly phase reduction and stabilization (OPRAS) routine utilizing the pulse shaper itself was developed. This routine automatically produces a compressed pulse with a minimized relative phase between the two polarization components. A phase stability of σ = 31.9 mrad (λ/197) over nearly 24 hours was measured by employing OPRAS. Various pulse sequences exceeding the capabilities of conventional pulse shapers were generated and characterized. The experimental results proved that shaped pulses with arbitrary phase, amplitude, and polarization states can be created. In all cases very high agreement between the target parameters and the experimental data was achieved.
For the future use of the setup also possible modifications were suggested. These are not strictly required, but all of them could further improve the performance and flexibility of the setup. Firstly, it was illustrated how a “dual-output” of the setup can be realized. With this modification it would be possible to use the main intensity of the shaped pulse for an experiment while using a small fraction to characterize the pulse or to perform OPRAS simultaneously. Secondly, the basic idea of replacing the telescope by focusing mirrors in order to eliminate the chromatic aberrations was presented. Regarding the different parameterizations for vector-field shaping, some modifications increasing the flexibility of the implemented bases and the realization of a von Neumann Basis for the presented setup were proposed. In future experiments, the vector-field shaper will be used in conjunction with a photoemission electron microscope (PEEM). This approach combines the temporal resolution provided by ultrashort laser pulses with the high spatial resolution gained by electron microscopy in order to perform two-dimensional spectroscopy and coherent control on nanostructures with polarization-shaped femtosecond laser pulses. In combination with other chiral-sensitive experimental setups implemented earlier in our group, the vector-field shaper opens up new perspectives for chiral femtochemistry and chiral control.
The designed vector-field shaper meets all requirements to generate high-precision polarization-shaped multipulse sequences. These can be used to perform numerous polarization-sensitive experiments. Employing the OPRAS routine, a quasi-infinitely long phase stability is achieved and complex and elaborated long-term measurements can be carried out. The fact that OPRAS demands no additional hardware and that only a single dual-layer LC SLM and inexpensive optics are required allows the building of a vector-field shaper at comparatively low costs. We hope that with the detailed insights into the optical design process as well as into the software implementation given in this thesis, vector-field shaping will become a standard technique just as conventional pulse shaping in the upcoming years.
In this work, functional plasmonic nanocircuitry is examined as a key of revolutionizing state-of-the-art electronic and photonic circuitry in terms of integration density and transmission bandwidth. In this context, numerical simulations enable the design of dedicated devices, which allow fundamental control of photon flow at the nanometer scale via single or multiple plasmonic eigenmodes. The deterministic synthesis and in situ analysis of these eigenmodes is demonstrated and constitutes an indispensable requirement for the practical use of any device. By exploiting the existence of multiple eigenmodes and coherence - both not accessible in classical electronics - a nanoscale directional coupler for the ultrafast spatial and spatiotemporal coherent control of plasmon propagation is conceived. Future widespread application of plasmonic nanocircuitry in quantum technologies is boosted by the promising demonstrations of spin-optical and quantum plasmonic nanocircuitry.
Optical antennas work similar to antennas for the radio-frequency regime and convert electromagnetic radiation into oscillating electrical currents. Charge density accumulations form at the antenna surface leading to strong and localized near-fields. Since most optical antennas have dimensions of a few hundred nanometers, their near-fields allow the focusing of electromagnetic fields to volumes much smaller than the diffraction limit, with intensities several orders of magnitude larger than achievable with classical diffractive and refractive optical elements. The task to maximize the emission of a quantum emitter, a point-like entity capable of reception and emission of single photons, is identical to the task to maximize the field intensity at the position of the quantum emitter. Therefore it is desirable to optimize the capabilities of focusing optical antennas.
Radio-frequency-antenna designs scaled to optical dimensions of several hundred nanometers show already a decent performance. However, optical frequencies lie near the plasma frequency of the metals used for optical antennas and the mass of electrons cannot be neglected anymore. This leads to new physical phenomena. Light can couple to charge density oscillations, yielding a so-called Plasmon. Effects emerge which have no equivalent in the very advanced field of radio-frequency-technology, e.g.~volume currents and shortened effective wavelengths. Additionally the conductivity is not infinite anymore, leading to thermal losses. Therefore, the question for the optimal geometry of a focusing optical antenna is not easy to answer. However, up to now there was no evidence that there exist better alternatives for optical antennas than down-scaled radio-frequency designs.
In this work the optimization of focusing optical antennas is based on an approach, which often proved successful for radio-frequency-antennas in complex applications (e.g.~broadband and isotropic reception): evolutionary algorithms. The first implementation introduced here allows a large freedom regarding particle shape and count, as it arranges cubic voxels on a planar, square grid. The geometries are encoded in a binary matrix, which works as a genome and enables the methods of mutation and crossing as mechanism of improvement. Antenna geometries optimized in this way surpass a comparable dipolar geometry by a factor of 2. Moreover, a new working principle can be deduced from the optimized antennas: a magnetic split-ring resonance can be coupled conductively to dipolar antennas, to form novel and more effective split-ring-antennas, as their currents add up constructively near the focal point.
In a next step, the evolutionary algorithm is adapted so that the binary matrices describe geometries with realistic fabrication constraints. In addition a 'printer driver' is developed which converts the binary matrices into commands for focused ion-beam milling in mono-crystalline gold flakes. It is shown by means of confocal two-photon photo-luminescence microscopy that antennas with differing efficiency can be fabricated reliably directly from the evolutionary algorithm. Besides, the concept of the split-ring antenna is further improved by adding this time two split-rings to the dipole-like resonance.
The best geometry from the second evolutionary algorithm inspires a fundamentally new formalism to determine the power transfer between an antenna and a point dipole, best termed 'three-dimensional mode-matching'. Therewith, for the first time intuitive design rules for the geometry of an focusing optical antenna can be deduced. The validity of the theory is proven analytically at the case of a point dipole in from of a metallic nano sphere.
The full problem of focusing light by means of an optical antenna can, thus, be reduced to two simultaneous mode-matching conditions -- on the one hand with the fields of a point dipole, on the other hand with a plane wave. Therefore, two types of ideal focusing optical antenna mode patterns are identified, being fundamentally different from the established dipolar antenna mode. This allows not only to explain the functionality of the evolutionary antennas and the split-ring antenna, but also helps to design novel plamonic cavity antennas, which lead to an enhanced focusing of light. This is proven numerically in direct comparison to a classical dipole antenna design.
In this thesis, the synthesis and photophysics of a great variety of squaraine dyes are presented. This variety is based on four parent squaraines containing either indolenine or quinoline heterocycles. By a suitable choice of the donor and acceptor unit, the optical properties can already be adapted to the properties desired on the stage of the monomer.
To promote a further derivatisation of these dyes, diverse functional groups are attached to the monomers using transition metal-catalysed C-C coupling reactions. However, this has to be preceded by the synthesis of bromine-functionalised derivatives as a direct halogenation of squaraine dyes is not feasible. Therefore, the halogen function is already introduced in precursor molecules giving rise to a molecular building block system containing bromine-, boronic ester-, and alkyne-functionalised monomer units, which pave the way to a plethora of squaraine oligomers and polymers.
The indolenine homopolymer pSQB-1 as well as the corresponding small molecular weight oligomers dSQB-1 and tSQB were synthesized applying Ni-mediated Yamamoto and Pd-catalysed Suzuki coupling methodologies, respectively. The motivation for this project relied on the fundamental investigations by Völker et al. on pSQB-V. A progressive red-shift of the lowest energy absorption maximum from the dimer to the polymer was observed in CHCl3 compared to the monomer. With increasing number of monomer units, the exciton coupling decreases from the dimer to the polymer. In addition, the shape of the absorption band manifold shows a strong dependence on the solvent, which was also observed by Völker et al. J-type aggregate behavior is found in chlorinated solvents such as CHCl3 and DCM, whereas H-type aggregates are formed in acetone. Temperature-dependent absorption studies in PhCN reveals a reversible equilibrium of diverse polymer conformers, which manifests itself in a gradual change from H-aggregate behavior to a mixture with a more pronounced J-aggregate behavior upon raising the temperature. It isassumed that both characteristic aggregate bands correlate in borderline cases with two polymer structures which can be assigned to a zig-zag and a helical structure. As no experimental evidence for these structures could hitherto be provided by NMR, TD-DFT computations on oligomers (22-mers) can reproduce very closely the characteristic features of the spectra for the two conformational isomers.
The subsequent chapters are motivated by the goal to influence the optical properties through a control of the superstructure and thus of the intramolecular aggregate formation.
On the one hand, bulky groups are implemented in the 3-position of the indolenine scaffold to provoke steric repulsion and thus favoring J-aggregate behavior at the expense of helical arrangements. The resulting homopolymer pDiPhSQB bearing two phenyl groups per indolenine exhibits J-type aggregate behavior with red-shifted absorption maxima in all considered solvents which is explained to be caused by the formation of elongated zig-zag structures. Furthermore, single-crystal X-ray analysis of monomer DiPhSQB-2-Br2 reveals a torsion of the indolenine moieties as a consequence of steric congestion. The twist of the molecular geometry and the resulting loss of planarity leads to a serious deterioration of the fluorescence properties, however a significant bathochromic shift of ca. 1 200 cm-1 of the lowest absorption band was observed compared to parent SQB, which is even larger than the shift for dSQB-1 (ca. 1 000 cm-1).
On the other hand, a partial stiffening of the polymer backbone is attempted to create a bias for elongated polymer chains. In this respect, the synthetic approach is to replace every second biarylaxis with the rigid transoid benzodipyrrolenine unit. Despite a rather low average degree of polymerization < 10, exclusively red-shifted absorption maxima are observed in all solvents used.
In order to complete the picture of intramolecular aggregates through the selective design of H-aggregates, a squaraine-squaraine copolymer was synthesised containing the classic cisoid indolenine as well as the cisoid quinoline building block. Taking advantage of the highly structure directing self-assembly character of the quinoline moiety, the copolymer pSQBC indeed showes a broad, blue-shifted main absorption band in comparison with the monomer unit dSQBC. The shape of the absorption band manifold solely exhibited a minor solvent and temperature dependence indicating a persistent H-aggregate behaviour. Hence, as a proof of concept, it is shown that the optical properties of the polymers (H- and J-aggregate) and the corresponding superstructure can be inherently controlled by an adequate design of monomer precursors.
The last chapter of this work deals, in contrast to all other chapters, with intermolecular aggregates. It is shown that the two star-shaped hexasquarainyl benzenes hSQA-1 and hSQA-2 exhibit a strong propensity for self-organisation. Concentration- and temperature-dependent studies reveal a great driving force for self-assembly in acetone. While the larger hSQA-2 instantaneously forms stable aggregates, the aggregates of hSQA-1 shows a pronounced kinetic stability. Taking advantage of the kinetic persistency of these aggregates, the corresponding kinetic activation parameters for aggregation and deaggregation can be assessed. The absorption spectra of both hexasquarainyl benzenes in the aggregated state reveal some striking differences. While hSQA-1 features an intensive, very narrow and blue-shifted absorption band, two red-shifted bands are observed for hSQA-2, which are closely located at the monomer absorption. The very small bandwidth of hSQA-1 are interpreted to be caused by exchange narrowing and pointed towards highly ordered supramolecular aggregates. The concentration-dependent data of the two hexasquarainyl benzenes can be fitted to the dimer-model with excellent correlation coefficients, yielding binding constants in excess of 10^6 M-1, respectively. Such high binding constants are very surprising, considering the unfavourable bulky 3,3-dimethyl groups of the indolenine units which should rather prevent aggregation. Joint theoretical and NMR spectroscopic methods were applied to unravel the supramolecular aggregate structure of hSQA-1, which is shown to consist of two stacked hexasquarainyl benzenes resembling the picture of two stacked bowls.
The focus of this work was the investigation of energy transfer between charge transfer states. For this purpose the multidimensional chromophores HAB-S, HAB-A, B1 and B2 were synthesised, each consisting of three electron donor and three electron acceptor redox centres linked symmetrically or asymmetrically by the hexaarylbenzene framework. Triarylamines represent in all these compounds the electron donors, whereas the electron poor centres were triarylboranes in B1 and B2 and PCTM centres in HAB-S and HAB-A, respectively. The hexaarylbenzenes were obtained by cobalt catalysed cyclotrimerisation of the respective tolan precursors. In addition, Star was synthesised, which consists of a central PCTM linked to three triarylamin centres by tolan bridging units in a star-like configuration. The hexaarylbenzene S1a/b substituted with six squaraine chromophores could not be realised. It is assumed that the cyclotrimerisation catalyst Co2(CO)8 does not tolerate the essential hydroxyl groups in the tolan precursor S2a. The alternative reaction pathway to execute the cyclotrimerisation reaction first and introduce the hydroxyl groups thereafter failed as well, because the required hexaarylbenzene substituted by six semisquaric acid moieties could not be synthesised. However, energy transfer interactions could be investigated in the tolan precursor S2a with two squaraine units to obtain information about the electronic coupling provided by the tolan bridge. For all multidimensional compounds model molecules were synthesised with only a single donor-acceptor pair (B3, Star-Model and HAB-Model). This allows a separate consideration of energy and charge transfer processes. It has to be stressed that in all before mentioned multidimensional compounds the “through bond” energy transfer interaction between neighbouring IV-CT states is identical to a transfer of a single electron between two redox centres of the same kind (e.g. TAA -> TAA+). The latter can be analysed by electron transfer theory. This situation is observed when the two IV-CT states transferring energy share one redox centre.
All compounds containing PCTM centres were characterised by paramagnetic resonance spectroscopy. Thereby, a weak interaction between the three PCTM units in HAB-S and HAB-A was observed. In addition, when oxidising Star-Model, a strongly interacting singlet or triplet state was obtained. In contrast, signals corresponding to a weakly interacting biradical were obtained for HAB-Model+. This indicates a strong electronic coupling between the redox centres provided by the tolan bridge and a weak coupling when linked by the hexaarylbenzene. This trend is supported by UV/Vis/NIR absorption measurements. The analysis of the observed IV-CT absorption bands by electron transfer theory reveals a weak electronic coupling of V = 340 cm-1 in HAB-Model and a distinctly stronger coupling of V = 1190-2900 cm-1 in Star-Model. In the oxidised HAB-S+, Star+ and Star-Model+ a charge transfer reversed from that of the neutral species, that is, from the PCTM radical to the electron poorer cationic TAA centre, was observed by spectroelectrochemistry. The temporal evolution of the excited states was monitored by ultrafast transient absorption measurements. Within the first picosecond stabilisation of the charge transfer state was observed, induced by solvent rotation. Anisotropic transient absorption measurements revealed that within the lifetime of the excited state (tau = 1-4 ps) energy transfer does not occur in the HABs whereas in the star-like system ultrafast and possibly coherent energy redistribution is observed. Taken this information together the identity between energy transfer and electron transfer in the specific systems were made apparent. It has to be remarked that neither energy transfer nor charge transfer theory can account for the very fast energy transfer in Star.
The electrochemical and photophysical properties of B1 and B2 were investigated by cyclic voltammetry, absorption and fluorescence measurements and were compared to B3 with only one neighbouring donor-acceptor pair. For the asymmetric B2 CV measurements show three oxidations as well as three reduction peaks whose peak separation is greatly influenced by the conducting salt due to ion-pairing and shielding effects. Consequently, peak separations cannot be interpreted in terms of electronic couplings in the generated mixed valence species. Transient absorption, fluorescence solvatochromism and absorption spectra show that charge transfer states from the amine to the boron centres are generated after optical excitation. The electronic donor-acceptor interaction is weak though as the charge transfer has to occur predominantly through space. The electronic coupling could not be quantified as the CT absorption band is superimposed by pi-pi* transitions localised at the amine and borane centres. However, this trend is in good agreement to the weak coupling measured for HAB-Model. Both transient absorption and fluorescence upconversion measurements indicate an ultrafast stabilisation of the charge transfer state in B1- B3 similar to the corresponding observations in HAB-S and Star. Moreover, the excitation energy of the localised excited charge transfer states can be redistributed between the aryl substituents of these multidimensional chromophores within fluorescence lifetime (ca. 60 ns). This was proved by steady state fluorescence anisotropy measurements, which further indicate a symmetry breaking in the superficially symmetric HAB. Anisotropic fluorescence upconversion measurements confirm this finding and reveal a time constant of tau = 2-3 ps for the energy transfer in B1 and B2. It has to be stressed that, although the geometric structures of B1 and HAB-S are both based on the same framework and furthermore the neighbouring CT states show in both cases similar Coulomb couplings and negligible “through bond” couplings, very fast energy transfer is observed in B1 whereas in HAB-S the energy is not redistributed within the excited state lifetime. To explain this, it has to be kept in mind that the energy transfer and the relaxation of the CT state are competing processes. The latter is influenced moreover by the solvent viscosity. Hence, it is assumed that this discrepancy in energy transfer behaviour is caused by monitoring the excited state in solvents of varying viscosity. Adding fluoride ions causes the boron centres to lose their acceptor ability due to complexation. Consequently, the charge transfer character in the donor-acceptor chromophores vanishes which could be observed in both the absorption and fluorescence spectra. However, the fluoride sensor ability of the boron centre is influenced strongly by the moisture content of the solvent possibly due to hydrogen bonding of water to the fluoride anions.
UV/Vis/NIR absorption measurements of S2a show a red-shift by 1800 cm-1 of the characteristic squarain band compared to the model compound S20. From exciton theory a Coulomb coupling of V = 410 cm-1 is calculated which cannot account for this strong spectral shift. Consequently, “through-bond” interactions have to contribute to the strong communication between the two squaraine chromophores in S2a. This is in accordance with the strong charge transfer coupling calculated for the tolan spacer in Star-Model.
This thesis describes novel concepts for the measurement of the static and dynamic properties of the electronic structure of molecules and nanocrystals in the liquid phase by means of coherent fluorescence-detected spectroscopy in two and three frequency dimensions. These concepts are based on the systematic variation ("phase cycling") of a sequence of multiple time-delayed femtosecond excitation pulses in order to decode a multitude of novel nonlinear signals from the resulting phase-dependent fluorescence signal. These signals represent any permutation of correlations between zero-, one-, two-, and three-quantum coherences. To this end, two new phase-cycling schemes have been developed which can simultaneously resolve and discriminate several nonlinear signals of sixth order, including those of the fourth order of nonlinearity.
By means of the sixth-order signals recorded in this work, static properties of highly excited electronic states in molecules such as their energies, transition dipole moments, and relative displacement of electronic potential surfaces, as well as dynamic properties in terms of their relaxation kinetics, can be ascertained. Furthermore, it was shown that these signals are suitable for the characterization of exciton-exciton correlations in colloidal quantum dots and for the measurement of ultrafast exciton-exciton annihilation in molecular aggregates.
The experiments performed in this thesis mark an important step towards the complete characterization of the nonlinear response of quantum systems. In view of this, the concept of fluorescence-detected multiple-quantum coherence multidimensional spectroscopy introduced here offers a unified, systematic approach.
In virtue of the technical advantages such as the use of a single excitation beam and the absence of nonresonant contributions, the measurement protocols developed here can be directly transferred to other incoherent observables and to sample systems in other states of matter. Furthermore, the approaches presented here can be systematically extended to higher frequency dimensions and higher orders of nonlinearity.
Time-resolved spectroscopy allows for analyzing light-induced energy conversion and
chromophore–chromophore interactions in molecular systems, which is a prerequisite in
the design of new materials and for improving the efficiency of opto-electronic devices.
To elucidate photo-induced dynamics of complex molecular systems, transient absorption
(TA) and coherent two-dimensional (2D) spectroscopy were employed and combined
with additional experimental techniques, theoretical approaches, and simulation models
in this work.
A systematic series of merocyanines, synthetically varied in the number of chromophores
and subsitution pattern, attached to a benzene unit was investigated in cooperation with
the group of Prof. Dr. Frank Würthner at the University of Würzburg. The global analysis
of several TA experiments, and additional coherent 2D spectroscopy experiments, provided
the basis to elaborate a relaxation scheme which was applicable for all merocyanine
systems under investigation. This relaxation scheme is based on a double minimum on the
excited-state potential energy surface. One of these minima is assigned to an intramolecular
charge-transfer state which is stabilized in the bis- and tris-chromophoric dyes by
chromphore–chromophore interactions, resulting in an increase in excited-state lifetime.
Electro-optical absorption and density functional theory (DFT) calculations revealed a
preferential chromophore orientation which compensates most of the dipole moment of
the individual chromophores. Based on this structural assignment the conformationdependent
exciton energy splitting was calculated. The linear absorption spectra of the
multi-chromophoric merocyanines could be described by a combination of monomeric and
excitonic spectra.
Subsequently, a structurally complex polymeric squaraine dye was studied in collaboration
with the research groups of Prof. Dr. Christoph Lambert and Prof. Dr. Roland Mitric
at the University of Würzburg. This polymer consists of a superposition of zigzag and
helix structures depending on the solvent. High-level DFT calculations confirmed the previous
assignment that zigzag and helix structures can be treated as J- and H-aggregates,
respectively. TA experiments revealed that in dependence on the solvent as well as the
excitation energy, ultrafast energy transfer within the squaraine polymer proceeds from
initially excited helix segments to zigzag segments or vice versa. Additionally, 2D spectroscopy
confirmed the observed sub-picosecond dynamics. In contrast to other conjugated
polymers such as MEH-PPV, which is investigated in the last chapter, ultrafast
energy transfer in squaraine polymers is based on the matching of the density of states
between donor and acceptor segments due to the small reorganization energy in cyanine-like
chromophores.
Finally, the photo-induced dynamics of the aggregated phase of the conjugated polymer
MEH-PPV was investigated in cooperation with the group of Prof. Dr. Anna Köhler at the University of Bayreuth. Our collaborators had previously described the aggregation of MEH-PPV upon cooling by the formation of so-called HJ-aggregates based on exciton
theory. By TA measurements and by making use of an affiliated band analysis distinct
relaxation processes in the excited state and to the ground state were discriminated. By
employing 2D spectroscopy the energy transfer between different conjugated segments
within the aggregated polymer was resolved. The initial exciton relaxation within the
aggregated phase indicates a low exciton mobility, in contrast to the subsequent energy
transfer between different chromophores within several picoseconds.
This work contributes by its systematic study of structure-dependent relaxation dynamics
to the basic understanding of the structure-function relationship within complex
molecular systems. The investigated molecular classes display a high potential to increase
efficiencies of opto-electronic devices, e.g., organic solar cells, by the selective choice of
the molecular morphology.
Within the framework of this thesis, photolysis reactions in the liquid phase were investigated by means of ultrafast optical spectroscopy. Apart from molecular studies dealing with the highly spin-dependent reactivity of diphenylcarbene (DPC) in binary solvent
mixtures and ligand dissociation reactions of so-called CO-releasing molecules (CORMs),
special emphasis was put on the implementation and characterization of methods improving
and extending the signal detection in conventional pump–probe transient absorption setups.
The assumption of DPC being an archetypal triplet-ground-state arylcarbene was recently questioned by matrix-isolation studies at low temperatures. DPC embedded in argon matrices revealed a hitherto unknown reactivity when the carbene environment was modified by small amounts of methanol dopant molecules. To complement these findings with liquid-phase experiments at room temperature, femtosecond pump–probe transient absorption spectroscopy with probing in the visible and ultraviolet regime was employed to unravel primary reaction processes of DPC in solvent mixtures. Supported by quantum chemical simulations conducted by our collaborators, it was shown that a competition between the reaction pathways occurs that not only depends on the solvent molecule near-by but also on its interaction with other solvent molecules. In-depth analysis of the solvation dynamics and the amount of nascent intermediates corroborates the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking analogy to complexes found at cryogenic temperatures.
Probing the transient absorption of molecules in the mid-infrared spectral range benefits from the high chemical specificity of molecules’ vibrational signatures. The technique of chirped-pulse upconversion (CPU) constitutes a promising alternative to standard direct multichannel MCT detection when accessing this spectral detection window. Hence, one chapter of this thesis is dedicated to a direct comparison between both detection methods. By conducting an exemplary pump–probe transient absorption experiment, it became evident, that the additional nonlinear interaction step is responsible for increased noise levels when using CPU. However, a correction procedure capable of removing these additional noise contributions—stemming from the fundamental laser radiation used for upconversion—was successfully tested. Perhaps most importantly for various spectroscopic applications, CPU scored with a significantly extended detection bandwidth owing to the high pixel numbers of modern CCD cameras.
Transition-metal complexes capable of releasing small molecular messengers upon photoactivation are promising sources of gasotransmitters such as carbon monoxide (CO) or nitric oxide (NO) in biological applications. However, only little is known about the characteristic time scales of ligand dissociation in this class of molecules. For this purpose, two complexes were investigated with femtosecond time resolution: [Mn(CO)3(tpm)]Cl with tpm=tris(2-pyrazolyl)methane, a manganese tricarbonyl complex which has proven to be selective and cytotoxic to cancer cells, and [Mo(CO)2(NO)(iPr3tacn)]PF6 with iPr3tacn=1,4,7-triisopropyl-1,4,7-triazacyclononane, a molybdenum complex containing both carbonyl and nitrosyl ligands. By conducting pump–probe transient absorption measurements in different spectral probing windows supported by quantum chemical calculations and linear absorption spectroscopy, it was shown that both complexes are able to release one CO ligand within the first few picoseconds after UV excitation. The results complement existing studies which focused on the molecules’ ligand-releasing properties upon long-term exposure. The additional information gained on an ultrafast time scale provides a comprehensive understanding of individual reaction steps connected with ligand release in this class of molecules. Hence, the studies might create new incentives to develop modified molecules for specific applications.
This work brings forward successful implementations of ultrafast chirality-sensitive spectroscopic techniques by probing circular dichroism (CD) or optical rotation dispersion (ORD). Furthermore, also first steps towards chiral quantum control, i.e., the selective variation of the chiral properties of molecules with the help of coherent light, are presented.
In the case of CD probing, a setup capable of mirroring an arbitrary polarization state of an ultrashort laser pulse was developed. Hence, by passing a left-circularly polarized laser pulse through this setup a right-circularly polarized laser pulse is generated. These two pulse enantiomers can be utilized as probe pulses in a pump--probe CD experiment. Besides CD spectroscopy, it can be utilized for anisotropy or ellipsometry spectroscopy also. Within this thesis, the approach is used to elucidate the photochemistry of hemoglobin, the oxygen transporting protein in mammalian blood. The oxygen loss can be triggered with laser pulses as well, and the results of the time-resolved CD experiment suggest a cascade-like relaxation, probably through different spin states, of the metallo-porphyrins in hemoglobin.
The ORD probing was realized via the combination of common-path optical heterodyne interferometric polarimetry and accumulative femtosecond spectroscopy. Within this setup, on the one hand the applicability of this approach for ultrafast studies was demonstrated explicitly. On the other hand, the discrimination between an achiral and a racemic solution without prior spatial separation was realized. This was achieved by inducing an enantiomeric excess via polarized femtosecond laser pulses and following its evolution with the developed polarimeter. Hence, chiral selectivity was already achieved with this method which can be turned into chiral control if the polarized laser pulses are optimized to steer an enhancement of the enantiomeric excess.
Furthermore, within this thesis, theoretical prerequisites for anisotropy-free pump--probe experiments with arbitrary polarized laser pulses were derived. Due to the small magnitude of optical chirality-sensitve signals, these results are important for any pump--probe chiral spectroscopy, like the CD probing presented in this thesis. Moreover, since for chiral quantum control the variation of the molecular structure is necessary, the knowledge about rearrangement reactions triggered by photons is necessary. Hence, within this thesis the ultrafast Wolff rearrangement of an α-diazocarbonyl was investigated via ultrafast photofragment ion spectroscopy in the gas phase. Though the compound is not chiral, the knowledge about the exact reaction mechanism is beneficial for future studies of chiral compounds.
Photoinduced processes are nowadays studied with a huge variety of spectroscopic methods. In the liquid phase, transient absorption spectroscopy is probably the most versatile pump–probe technique used to study light-induced molecular phenomena. Optical time-resolved spectroscopy is established in a large number of laboratories and is still further being developed with respect to many technical aspects. Nevertheless, the full potential of shortening the data-acquisition time—necessary for the investigation of rapidly photodegrading samples and observation of macroscopically fast processes—achievable with high-repetition-rate laser systems and shot-to-shot detection was not fully exploited. Especially, shot-to-shot detection is highly beneficial due to the high correlation of subsequent laser pulses.
The development and implementation of 100 kHz broadband shot-to-shot data acquisition was presented in Chapter 3. For an established laser dye as a benchmark system, ultrafast excited-state dynamics were measured for the first time with broadband shot-to-shot detection at 100 kHz. An analysis of both the noise characteristics of the employed laser and the correlation of subsequent pulses quantified the advantage of shot-to-shot data acquisition. In the utilized software environment, the time for measuring a complete data set could be sped up by a factor of three or even higher compared to a laser system working at 1 kHz. So far, the limiting factor is the data processing and the movement of the mechanical delay stage. Nevertheless, the new shot-to-shot detection has the potential to shorten the measurement time up to a factor of 100. The data quality is improved by a factor of three when the hitherto conventional averaging scheme is compared to shot-to-shot acquisition for the same number of laser pulses. The expansion of shot-to-shot data acquisition for high repetition rates will allow studies on sensitive samples as exposure times can strongly be reduced to achieve the same signal-to-noise ratio. In addition, multidimensional spectroscopy can also be extended to high-repetition shot-to-shot readout allowing an efficient recording of data. Therefore, in future experiments, dynamics and couplings in sensitive samples and kinetic processes could be studied in more detail.
Complex photophysical and photochemical phenomena are subject of many fields of research. Many of these multifaceted processes are not yet fully understood. Therefore, a possible approach is the elucidation of single reaction steps with the combination of transient absorption spectroscopy and a suitable, less complex model system. The systematic variation of the model system’s properties and environments, e.g., by chemical substitution or adequate choice of the solvent allows the determination of essential entities and reactivities thereof. Proper knowledge of an individual intermediate step and its determining factors can enhance the understanding of the complete photoreaction process.
The application of transient absorption spectroscopy was shown for the optically-induced electron transfer in a series of donor–acceptor oligomers in Chapter 4. In general, the solvent relaxation times were isolated from the back-electron-transfer dynamics by a global lifetime analysis. For the smallest oligomeric structure where complete charge separation is possible, an ultrafast equilibration leads to charge recombination from the configuration showing the lowest barrier for recombination. The back-electron transfer strongly depends on the utilized solvent. Whereas in dichloromethane the back-electron transfer occurs with the maximum rate in the barrierless optimal region, the dynamics in toluene are governed by a Marcus inverted-region effect. The experimentally observed rates were also estimated by theoretical calculations of the respective barriers. The study did not only successfully unravel charge transfer in the oligomeric systems but also improved the understanding of the electron-transfer properties of larger polymers from an earlier study. Therefore, the combination of length variation and time-resolved spectroscopy is an important step towards the correct prediction of charge-carrier dynamics in macroscopic devices, e.g., for photovoltaics.
The bond dissociation of a carbon-monoxide-releasing molecule in aqueous solution was studied in Chapter 5 as a prototype reaction for the photo-triggered breaking of a bond. It was shown that upon excitation only one carbon-monoxide ligand of the tricarbonyl complex is dissociated. A fraction of the photolyzed molecules restore the intact initial complex by geminate recombination within the temporal resolution of the experiment. However, the recombination could be detected by the hot ground-state infrared absorption of the complex. The detectable dicarbonyl formed upon CO release distributes excess energy from the absorbed photon into low-frequency modes which result in broadened absorption bands like for the recombined tricarbonyl. The free coordination site in the ligand sphere is filled with a solvent water molecule. Despite numerous studies of metal carbonyls studied in alkaneous solutions, the elucidation of the dynamics of a CORM in aqueous solution added another important detail to the photochemistry of this class of compounds. Experiments employing a second ultraviolet pump pulse did not trigger further CO dissociation and hence no formation of a monocarbonyl species; this might either be due to a different release mechanism without a further photochemical step or a strong spectral shift of the dicarbonyl’s absorption. Both reasons could explain why degenerate pump–repump–probe spectroscopy is inefficient. However, further experiments with ultraviolet probe pulses could substantiate whether the intermediate dicarbonyl reacts further photochemically or not. Apart from the model-system character of the CORM for bond dissociation, the study could determine exactly how many CO ligands are initially photolyzed off. Detailed knowledge of the release mechanism will affect the previous use and application as well as the further development of CORMs as therapeutic prodrugs to deliver high local concentrations of CO in cancerous or pathological tissue. Hence, the study of two-photon absorption properties which are important for in vivo applications of CORMs should be the main focus in further spectroscopic experiments.
In Chapter 6, both abovementioned molecular phenomena—electron transfer and bond dissociation—were studied in combination. The photochemistry of a tetrazolium salt was studied in detail in a variety of different solvents. Being a relatively small molecule, the studied tetrazolium cation shows a multifaceted photochemistry and is therefore a textbook example for the combination of ultrafast molecular phenomena studied in different environments. Within femtoseconds, the tetrazolium ring is opened. The biradicalic species is then reduced via uptake of an electron from the solvent. The formation of the ring-open formazan photoproduct from this point of the reaction sequence on was excluded by experiments with acidic pH value of the solution. The ring-open radical is stabilized by ring-closure. The resulting tetrazolinyl radical was already observed in experiments with microsecond time resolution. However, its formation was observed in real time for the first time in this study. Irradiation of a tetrazoliumsalt solution yields different photoproduct distributions depending on the solvent. However, it was shown that all photoproducts have a tetrazolinyl radical as a common precursor on an ultrafast time scale. In combination with studies from the literature, the complete photochemical conversion of a tetrazolium salt was clarified in this study. Apart from the prototype character of the reaction sequence, the reaction mechanism will have impact on research associated with life science where tetrazolium assays are used on a daily basis without taking into account of photochemical conversion of the indicating tetrazolium ion and its photochemically formed reactive intermediates. On the basis of the tetrazolium-ion photochemistry, the rich photochemistry of the formazan photoproduct, including structural rearrangements and subsequent reformation of the tetrazolium ion, might be the subject of future studies.
This thesis shows a method advancement and application of transient absorption spectroscopy to exemplary molecular model systems. The insights into each respective field did not only enlighten singular aspects, but have to be seen in a much larger context. Understanding complex photoinduced processes bottom-up by learning about their constituting steps—microscopically and on an ultrafast time scale—is an ideal method to approach understanding and prediction of phenomena in large molecular systems like biological or artificial architectures as for example used in photosynthetic light-harvesting and photovoltaics.
Pulse-Sequence Approaches for Multidimensional Electronic Spectroscopy of Ultrafast Photochemistry
(2014)
Observing chemical reactions in real time with femtosecond laser pulses has evolved into a very popular field of research since it provides fascinating insights into the nature of photochemical transformations. Nevertheless, many photochemical reactions are still too complex for which reason the underlying mechanisms and all engaged species cannot be identified thoroughly. In these cases, conventional time-resolved spectroscopy techniques reach their technical limits and advanced approaches are required to follow the conversion of reactants to their products including all reaction intermediates.
The aim of this work was therefore the development of novel methods for ultrafast spectroscopy of photoreactive systems. Though the concept of coherent multidimensional spectroscopy has so far exclusively been used to explore photophysical phenomena, it also offers great potential for the study of photochemical processes due to its capability of extracting spectroscopic information along several frequency dimensions. This allows resolving the photochemical connectivity between various interconvertible molecular species with ultrafast temporal resolution on the basis of their absorption and emission properties as the spectral correlations are explicitly visualized in the detected spectra.
The ring-open merocyanine form of the photochromic compound 6-nitro BIPS was studied in Chap. 4 of this work. Merocyanines and their associated ring-closed spiropyrans are promising candidates for future applications as, for instance, molecular electronics or optical data storage due to their unique property of being switchable between two stable congurations via light illumination. Transient absorption with sub-50 fs temporal resolution and broadband probing was employed to characterize the photodynamics of this system with variable excitation wavelengths. Using global data analysis, it could be inferred that two different merocyanine isomers with differing excited-state lifetimes exist in solution. These isomers differ in the cis/trans conguration in the last bond of the methine bridge. The minority of isomers exist in the all-trans conguration (TTT) while the isomer with a cis conguration of the third dihedral angle (TTC) is dominant. A characteristic band, detected after long pump-probe delays, was attributed to the unidirectional cis->trans photoisomerization reaction of the TTC to the TTT form. The quantum yield of the reaction was estimated to be (18+-4) %. In addition, pronounced coherent vibrational wave-packet oscillations were observed and it was concluded that these signatures are related to the product formation.
Coherent two-dimensional electronic spectroscopy was successfully implemented using a partially collinear pump-probe beam geometry in combination with a femtosecond pulse shaper. The use of a whitelight probe continuum enabled us to probe contributions far-off the diagonal over the complete visible range. By properly adjusting the relative phase between the first two laser pulses with the pulse shaper, the principle of phase-cycling was explained and it was demonstrated that the measurement can be carried out in the so-called "rotating frame" in which the observed frequencies detected during the coherence time are shifted to lower values. It was shown that these concepts allow the extraction of the desired background-free photon echo while the amount of necessary data points is highly reduced.
In order to put our proposal of multidimensional spectroscopy of photoreactive systems into practice, third-order two- and three-dimensional spectroscopy was then employed for an in-depth analysis of a photoreactive process, in which the photoisomerization of 6-nitro BIPS served as a model system. The measured two-dimensional spectra revealed the cis->trans photoisomerization after long population times. By collecting a large data set of two-dimensional spectra for short population times and by applying a Fourier transform along the population time axis, the third-order three-dimensional spectrum was obtained. The novelty of this approach compared to coherent two-dimensional spectroscopy is the introduction of a third axis associated with the vibrational frequencies of the molecular system. In this way, the formation of the reaction product was evidenced and it was shown that the product is formed in its first excited singlet state within 200 fs after excitation. This method hence visualizes the photochemical connections between different reactive molecular species in an intuitive manner and further exposes the normal modes connecting reactant and product. Such conclusions cannot be drawn with conventional third-order techniques such as transient absorption since they are
not capable of capturing the full third-order response, but only a subset of it. The reaction mechanism and the role of the observed vibrational modes were uncovered by comparing the experimental data with the results of high-level quantum-chemical calculations performed by our collaborators in the group of Prof. B. Engels from the
theoretical chemistry department at the University of Würzburg. Specific calculated molecular normal modes could be assigned to the experimentally observed vibrational frequencies and potential energy surfaces of the electronic ground state and of the first excited state were computed. The technique implemented in this chapter is general and is applicable for the time-resolved analysis of a wide range of chemical reaction networks.
In the first part of Chap. 5, coherent two-dimensional spectroscopy was employed to track the reaction paths of the related 6,8-dinitro BIPS after S1 excitation. Several differences to the photochemical properties of 6-nitro BIPS were found. From the 2D spectra, the cis-trans isomerization between the two merocyanine isomers could be excluded as a major reaction path for this compound. To explore the dynamics after reexcitation to higher-lying electronic states, pump-repump-probe spectroscopy was implemented and the formation of a new species, a radical cation, was observed. To identify the precursor isomer, triggered-exchange two-dimensional spectroscopy, a fifth-order technique previously only available in the infrared regime for vibrational transitions, was implemented for the first time for electronic excitations in the visible. This approach combines the properties of the pump-repump-probe technique with the potential of coherent two-dimensional spectroscopy. It correlates the absorption frequency of a reactive molecular species with the emission signatures of the product formed from this species after an additional absorption of a photon. Using this method, it was unambiguously proven that only the TTC isomer reacts to the radical cation thus forming the precursor species of the reaction. Electronic triggered-exchange two-dimensional spectroscopy is hence another improved technology for time-resolved spectroscopy with applications in the study of multistep photoreactions and higher-lying electronic states. While in the two preceding chapters third- and fifth-order experiments were discussed that neglect the vectorial character of light-matter interactions, Chap. 6 focused on a novel theoretical formalism enabling the description of light fields optimized for polarization-sensitive higher-order nonlinearities. This formalism is based on the von Neumann time-frequency representation of shaped femtosecond laser pulses which permits the definition of multipulse sequences on a discrete time-frequency lattice. Hence, not only the temporal spacing between subpulses is adjustable, but also the center frequencies may be adapted such that they fit the experimental requirements. This method was generalized to the description of pulse sequences with time-varying polarization states. It was shown that by using this description, the polarization ellipticity, orientation angle, relative phase and intensity, and the time-frequency location of each subpulse is explicitly controllable. The accuracy of the transformations from Fourier space to von Neumann domain and vice versa was demonstrated. Moreover, a strict accordance between the von Neumann polarization parameters with the conventional parameters in time domain was found for well separated subpulses. A potential future application of this approach is polarization-sensitive multidimensional spectroscopy in which hidden cross peaks may be isolated by defining the pulses in the von Neumann picture with suitable polarization sequences. This method could also be used in quantum control experiments in which the polarization of the light field is used as a major control knob.
This thesis summarizes our efforts to open the field of femtochemistry to the concept of coherent multidimensional electronic spectroscopy. Making use of femtosecond pulse shaping, sub-50 fs temporal resolution, broadband spectral probing, higher-order nonlinearities, and new types of laser pulse descriptions, the presented methods might stimulate further future advancements in this research area.
The experimental technique predominantly employed within the scope of this Thesis constitutes one subarea of femtochemistry: the time-resolved spectroscopy of photoin-
duced chemical reactions in the liquid phase by means of molecular signatures in the mid-infrared (MIR) spectral range. Probing transient vibrational states, i.e., dynamic
changes in the vibrational motion of specic molecular subunits or functional Groups allows for a distinct separation and assignment of measured signals to emerging molecular species. For this purpose, one key building block is indispensable, which most of the investigations carried out within the eld of femtochemistry have in common: a coherent light source delivering ultrashort laser pulses with a temporal duration that matches the femtosecond time scale on which molecular motions typically occur. This instrumentation enables the observation of photoinduced chemical reactions from the
starting point|the excitation event to the appearance of intermediates to the nal formation of stable photoproducts after several pico- or nanoseconds.
This work comprises the acquisition and presentation of time-resolved spectroscopic data related to promising molecular systems upon photoexcitation as well as the im-
plementation and testing of experimental optical techniques both for the presented experiments but as well for experiments conceivable in the future. In addition, linear spectroscopy measurements and quantum-chemical simulations on the emerging chemical species have been carried out. In so doing, the primary processes and subse-
quently emerging reaction products of two compounds on a timescale of several nanoseconds after photoexcitation have been elucidated in great detail. Both compounds, the
[Mn(CO)3(tpm)]+ (tpm = tris(2-pyrazolyl)methane) CO-releasing molecule (CORM) and the 5-diazo Meldrum's acid (DMA), are of academic interest but in addition belong
to molecular classes that might be utilized in the near future as dark-stable prodrugs under physiological conditions or that are already utilized in industrial chemistry procedures, respectively. The ndings of both studies gave rise to implement and examine two techniques for prospective transient absorption experiments, namely the shaping and characterization of ultraviolet (UV) laser pulses and the recording of two-photon excitation spectra. Beyond that, since each of the depicted experiments is based on the detection of weak transient absorption signals in the MIR spectral region, two dif-
ferent detection schemes, via chirped-pulse upconversion (CPU) on the one hand and via direct multichannel MCT detection on the other hand, have been juxtaposed at the
conclusion of this work. Since both techniques are suitable in femtosecond pump-probe measurements but thereby exhibit individual strengths and weaknesses, a comparative study provides clarication of the respective pros and cons. The first study introduced within this work investigates the complex photochemistry
of DMA, a photoactive compound used in lithography and industrial chemistry. By femtosecond MIR transient absorption spectroscopy covering several nanoseconds, the
light-induced dynamics and ultrafast formation of several photoproducts from the manifold of reaction pathways have been disclosed to form a coherent picture of the overall
reaction scheme. After UV excitation of DMA dissolved in methanol to the second excited state S2, 70% of excited molecules relax back to the S0 ground state. In compet-
ing processes, they can either undergo an intramolecular Wolff rearrangement to form ketene, which reacts with a solvent molecule to an enol intermediate and further to carboxylate ester, or they rst relax to the DMA S1 state, from where they can isomerize to a diazirine. The third competing reaction channel, having the lowest quantum efficiency with respect to the rst two channels, is the formation of a singlet carbene out of the S1 state. From there an ylide can arise or, via an intersystem crossing, the triplet form of the carbene follows. Whereas the primary reaction steps occur on a picosecond timescale, the subsequently arising intermediates and stable photoproducts are formed
within a few hundreds to thousands of picoseconds. For a reliable identication of the involved compounds, density functional theory calculations on the normal modes and
Fourier-transform infrared spectroscopy of the reactant and the photoproducts in the chemical equilibrium accompany the analysis of the transient spectra. Additional experiments in ethanol and isopropanol led to slight spectral shifts as well as elongated time
constants due to steric hindrance in transient spectra connected with the ester Formation channel, further substantiating the assignment of the occurring reaction pathways and photoproducts.
The study demonstrated that the combination of linear and time-resolved spectroscopic measurements in conjunction with quantum-chemical calculations constitutes a powerful tool to unravel even highly complex photoreactions exhibiting multiple consecutive intermediate states within parallel reaction pathways. Although some of the individual reaction steps, for example the ketene formation via Wolff rearrangement, have been observed on ultrashort time scales before, this work encompassed the Observation of the whole set of appearing photoproducts of DMA in different alcohol solutions within several nanoseconds. In this sense, the ultrafast photochemistry of DMA represents a prototype example for a multisequential reaction scheme, elucidated by the capabilities of femtosecond MIR spectroscopy.
With a modified instrumentation concerning amongst others the system delivering the fundamental laser pulses or the generation of the UV pump pulses, the next ob-
jective within this work was to elucidate the primary processes upon UV Irradiation of a manganese tricarbonyl CORM in aqueous environment. The time-resolved
experiment was performed with two different pump wavelengths and furthermore supported by linear spectroscopy methods and time-dependent density functional theory (TDDFT) calculations on the excited states as well as DFT calculations on the ground
states. The measurements revealed that irradiating the compound with UV excitation pulses primarily leads to ultrafast photolysis of one CO ligand. Geminate recombination may occur within one picosecond but it remains a minor process as the photolyzed CO
group is liberated and the unoccupied coordination site is predominantly filled by an incoming solvent molecule. There was no evidence for hot CO bands, i.e., the remaining CO ligands|in the dicarbonyl photoproduct as well as in the intact CORM are not vibrationally excited through the UV excitation of the CORM. According to this, the excess energy merges into low-frequency vibrational modes associated with the molecule as a whole. Since studies on a macroscopic scale at irradiation times of several minutes prove that UV irradiation eventually leads to the release of two or even all three CO
ligands, further loss of CO most likely necessitates manganese oxidation or another interaction with light. To clarify the latter, a consecutive UV pulse was employed in order to excite the photoproducts subsequent to the initial pump interaction. However, the data obtained was not instructive enough to denitely exclude the manganese oxidation being responsible for the loss of further CO groups. Besides the exchange of a CO Group by a solvent molecule or the geminate recombination, the employment of two different excitation wavelengths in combination with ndings derived from the TDDFT calculations suggested another reaction process, namely the possibility that the excitation does not lead to any bond cleavage at all. As the CORM under investigation is tissue-selective and cytotoxic against cancer cells, knowledge of these rst photoinduced reaction steps is essential for a full understanding of its biological activity. Inspired by these two studies, experimental techniques for prospective transient absorption measurements have been implemented and tested within preparative measure-
ments. First, in the course of a UV-pump-MIR-probe experiment with specically tailored excitation pulses, one could pursue the aim of coherently controlling the outcome of a photoreaction in the liquid phase. Out of the rich photochemistry of DMA the vibrational signature of a particular molecular species might thereby serve as a feedback signal, which is a central part of a learning loop that adaptively determines the pulse shape that steers the quantum mechanical system upon photoexcitation into a desired direction. This motivated the installation and testing of devices by means of which the shaping and characterization of ultrashort laser pulses in the UV could be performed. Second, motivated by the biological applications of CORMs, one can imagine a scenario where a certain amount of CORMs is deposited inside cancerous tissue. Since the activation of CO loss by means of UV pulses is not possible due to the absorption characteristics of biological tissue, the simultaneous excitation via two photons from the visible spectral regime seems appealing. However, success or failure of such an application depends on whether the deposited compound efficiently absorbs two photons simultaneously, i.e., whether the two-photon absorption cross section is large enough. Therefore, a setup to record two-photon excitation spectra under full consideration of
the crucial laser pulse parameters like the pulse duration, energy and central wavelength was arranged and tested. The rst results were obtained with a commercially available reference system (Mn2CO10) but the setup as well as the described measurement and
data analysis procedure can easily be applied to record the two-photon absorption cross section of more promising molecular systems. Third, as the detection of probe pulses
in the MIR spectral region is part of each time-resolved measurement throughout this thesis, a comparison between the newly established technique of CPU and direct multi-
channel MCT detection is presented by means of pump{probe experiments on Mn2CO10 and Co4CO12 with a 1 kHz shot-to-shot data acquisition. It was shown that the CPU detection technique scores with its high spectral resolution and coverage of the easy-to-handle and more cost-effective CCD detectors. On the other hand, in the course of the additional nonlinear upconversion process intensity fluctuations of the chirped fundamental pulses are transferred to the probe spectrum in the visible regime. This entails a lower signal-to-noise ratio than the direct MCT detection, which can be compensated by an additional normalization procedure applied to the CPU probe pulses. As a consequence, the CPU detection scheme offers more flexibility for future investigations
employing MIR probe pulses. This is of great importance for many applications within the presented eld of femtochemistry as a huge variety of time-resolved investigations on a multitude of systems in the liquid phase is based on the detection of weak transient
absorption signals in the MIR spectral region.
In this work, femtosecond laser pulses are used to launch optical excitations on different nanostructures. The excitations are confined below the diffraction limit and propagate along the nanostructures.
Fundamental properties of these ultrashort optical near fields are determined by characterizing the far-field emission after propagation with a setup developed for this task. Furthermore, control of the nanooptical excitations' spatial and temporal evolution is demonstrated for a designed nanostructure.
Plasmonic nanostructures are considered promising candidates for essential components of integrated quantum technologies because of their ability to efficiently localize broad-band electromagnetic fields on the nanoscale. The resulting local near field can be understood as a spatial superposition of spectrally different plasmon-polariton modes due to the spectrally broad optical excitation, and thus can be described as a classical wave packet. Since plasmon polaritons, in turn, can transmit and receive non-classical light states, the exciting question arises to what extent they have to be described as quantum mechanical wave packets, i.e. as a superposition of different quantum states.
But how to probe, characterize and eventually manipulate the quantum state of such plasmon polaritons? Up to now, probing at room temperatures relied completely on analyzing quantum optical properties of the corresponding in-going and out-going far-field photon modes. However, these methods so far only allow a rather indirect investigation of the plasmon-polariton quantum state by means of transfer into photons. Moreover, these indirect methods lack spatial resolution and therefore do not provide on-site access to the plasmon-polariton quantum state. However, since the spectroscopic method of coherent two-dimensional (2D) nanoscopy offers the capability to follow the plasmon-
polariton quantum state both in Hilbert space and in space and time domain a complete characterization of the plasmon polariton is possible.
In this thesis a versatile coherent 2D nanoscopy setup is presented combining spectral tunability and femtosecond time resolution with spatial resolution on the nanometer scale due to the detection of optically excited nonlinear emitted electrons via photoemission electron microscopy (PEEM). Optical excitation by amplitude- and phase-shaped, systematically-modified and interferometric-stable multipulse sequences is realized, and characterized via Fourier-transform spectral interferometry (FTSI). This linear technique enables efficient data acquisition in parallel to a simultaneously performed experiment. The full electric-field reconstruction of every generated multipulse sequence is used to analyze the effect of non-ideal pulse sequences on the two-dimensional spectral data of population-based multidimensional spectroscopy methods like, e.g., the coherent 2D nanoscopy applied in this thesis. Investigation of the spatially-resolved nonlinear electron emission yield from plasmonic gold nanoresonators by coherent 2D nanoscopy requires a quasi-particle treatment of the addressed plasmon-polariton mode and development of a quantum model to adequately describe the plasmon-assisted multi-quantum electron emission from nanostructures. Good agreement between simulated and experimental data enables to connect certain spectral features to superpositions of non-adjacent plasmon-polariton quantum states, i.e, non-adjacent occupation-number states of the underlying quantized, harmonic oscillator, thus direct probing of the plasmon-polariton quantum wave packet at the location of the nanostructure.
This is a necessary step to locally control and manipulate the plasmon-polariton quantum state and thus of general interest for the realization of nanoscale quantum optical devices.
The goal of this thesis was the development and application of higher-order spectroscopic techniques. In contrast to ordinary pump–probe (PP) and two-dimensional (2D) spectroscopy, higher-order coherently detected spectroscopic methods measure a polarization that has an order of nonlinearity higher than three. The key idea of the techniques in this thesis is to isolate the higher-order signals from the lower-order signals either by their excitation frequency or by their excitation intensity dependence. Due to the increased number of interactions in higher-order spectroscopy, highly excited states can be probed. For excitonic systems such as aggregates and polymers, the fifth-order signal allows one to directly measure exciton–exciton annihilation (EEA). In polymers and aggregates, the exciton transport is not connected to a change of the absorption and can therefore not be investigated with conventional third-order techniques. In contrast, EEA can be used as a probe to study exciton diffusion in these isonergetic systems. As a part of this thesis, anisotropy in fifth-order 2D spectroscopy was investigated and was used to study geometric properties in polymers.
In 2D spectroscopy, the multi-quantum signals are separated from each other by their spectral position along the excitation axis. This concept can be extended systematically to higher signals. Another approach to isolate multi-quantum signals in PP spectroscopy utilizes the excitation intensity. The PP signal is measured at specific excitation intensities and linear combinations of these measurements result in different signal contributions. However, these signals do not correspond to clean nonlinear signals because the higher-order signals contaminate the lower-order multi-quantum signals. In this thesis, a correction protocol was derived that uses the isolated multiquantum signals, both from 2D spectroscopy and from PP spectroscopy, to remove the contamination of higher-order signals resulting in clean nonlinear signals. Using the correction on the third-order signal allows one to obtain annihilation-free signals at high excitation intensities, i.e., with high signal-to-noise ratio. Isolation and correction in PP and 2D spectroscopy were directly compared by measuring the clean third-order signals of squaraine oligomers at high excitation intensities. Furthermore, higher-order PP spectroscopy was used to isolate up to the 13th nonlinear order of squaraine polymers.
The demonstrated spectroscopic techniques represent general procedures to isolate clean signals in terms of perturbation theory. The technique of higher-order PP spectroscopy needs only small modifications of ordinary PP setups which opens the field of higher-order spectroscopy to the broad scientific community. The technique to obtain clean nonlinear signals allows one to systematically increase the number of interacting (quasi)particles in a system and to characterize their interaction energies and dynamics.
Coherent Multidimensional Spectroscopy in Molecular Beams and Liquids Using Incoherent Observables
(2018)
The aim of the present work was to implement an experimental approach that enables coherent two-dimensional (2D) electronic spectroscopy of samples in various states of matter. For samples in the liquid phase, a setup was realized that utilizes the sample fluorescence for the acquisition of 2D spectra. Whereas the liquid-phase approach has been established before, coherent 2D spectroscopy on gaseous samples in a molecular beam as developed in this work is in fact a new method. It employs for the first time cations in a time-of-flight mass spectrometer for signal detection and was used to obtain the first ion-selective 2D spectra of a molecular-beam sample. Additionally, a new acquisition concept was developed in this thesis that significantly decreases measurement times in 2D spectroscopy using optimized sparse sampling and a compressed-sensing reconstruction algorithm.
Characteristic for the variant of 2D spectroscopy presented in this work is the usage of a phase-coherent sequence of four laser pulses in a fully collinear geometry for sample excitation. The pulse sequence was generated by a custom-designed pulse shaper that is capable of rapid scanning by changing the pulse parameters such as time delays and phases with the repetition rate of the laser. The sample's response was detected by monitoring incoherent observables that arise from the final-state population, for instance fluorescence or cations. Phase cycling, i.e., signal acquisition with different combinations of the relative phases of the excitation pulses, was applied to extract nonlinear signal contributions from the full signal during data analysis.
Liquid-phase 2D fluorescence spectroscopy was established with the laser dye cresyl violet as a sample molecule, confirming coherent oscillations previously observed in literature that are originating from vibronic coherences in specific regions of the 2D spectrum.
The data set of this experiment was used subsequently to introduce optimized sparse sampling in 2D spectroscopy. An optimization algorithm was implemented in order to find the best sampling pattern while taking only one quarter of the regular time-domain sampling points, thereby reducing the acquisition time by a factor of four. Signal recovery was based on a new and compact representation of 2D spectra using the von Neumann basis, which required about six times less coefficients than the Fourier basis to retain the relevant information. Successful reconstruction was shown by recovering the coherent oscillations in cresyl violet from a reduced data set.
Finally, molecular-beam coherent 2D spectroscopy was introduced with an investigation of ionization pathways in highly-excited nitrogen dioxide, revealing transitions to discrete auto-ionizing states as the dominant contribution to the ion signal. Furthermore, the advantage of the time-of-flight approach to obtain reactant and product 2D spectra simultaneously enabled the observation of distinct differences in the multiphoton-ionization response functions of the nitrogen dioxide cation and the nitrogen oxide ionic fragment.
The developed experimental techniques of this work will facilitate fast acquisition of 2D spectra for samples in various states of matter and permit reliable direct comparison of results. Therefore, they pave the way to study the properties of quantum coherences during photophysical processes or photochemical reactions in different environments.