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Reactive hydrocarbon species are important in a multitude of different scientific areas. In this thesis, the vibrational spectra of hydrocarbon radicals, biradicals and their reaction product have been studied in a gas-phase environment. The specific molecules investigated here, are of particular importance in the field of combustion and astrochemistry. They were produced from suitable precursors in a pyrolytically heated micro-reactor and subsequently seeded in an appropriate carrier gas. As methodology, IR/UV ion dip spectroscopy has been utilized, which delivers massselected gas-phase IR spectra of all ionizable species detectable in the molecular beam. These, with the help of DFT calculations, allow for determination of the fingerprint IR spectra, identification of mass carriers and formulation of potential reaction mechanisms. All studies have been conducted in collaboration with the group of Prof. Dr. Anouk M. Rjis and the necessary potent IR radiation has been provided by the free-electron laser FELIX. Thus, the IR/UV measurements have been executed at the FELIX Laboratory of the Radboud University in Nijmegen. The first study presented in this thesis is the investigation of ortho-benzyne in Chapter 3.1. This molecule is of particular interest due to its uncommon electronic structure and its role in high-temperature reactions. Although, the infrared spectrum of o-C6H4 was not accessible, a number of reaction products were identified via their fingerprint spectra. Masses in the range from 78 - 228 were assigned to their respective carrier. The identified species include typical PAHs like naphthalene, phenanthrene, up to triphenylene. The identified masses further suggest a PAH growth heavily influenced by diradical 1,4-cycloaddition followed by fragmentation, as well as by classical HACA- and PAC-like mechanisms. These results were augmented by threshold photoionization measurements from Engelbert Reusch, who identified lighter reaction products, which have insufficient IR absorption or unsuitable ionization characteristics to be identified in the IR/UV experiment. An interesting observation is the identification of m/z = 152. This carrier has been assigned differently by the IR and TPES experiments. Whereas the IR spectrum clearly identifies the species as 2-ethynylnaphthalene, the TPES evidently is in great agreement with biphenylene. This is a good example how different experimental methodologies can benefit from each other to gain a deeper insight into the actual science of a particular system. Probably, the prime example for an aromatically resonance stabilized radical is benzyl. This radical is of high importance for many combustion studies, as it represents the primary high-temperature decomposition product of toluene. The goal of the study was the identification of the benzyl self reaction products and the results are discussed in Section 3.2. The radical was pyrolytically produced by its respective nitrite precursor. The mass spectrum showed that the benzyl self reaction formed two products with C11 and three with C14 constitution. All mass peaks were evenly spaced by two mass units, respectively, which suggests a close relation in formation. Indeed, the C11 products were identified as diphenylmethane and fluorene, which are simply connected via cyclization. The heaviest product was identified as phenanthrene, which is formed via the cyclization of bibenzyl to 9,10-dihydrophenanthrene and subsequent elimination of hydrogen. This result was quiet interesting as the intermediate of this reaction was often assumed to be stilbene, which was not observed in the study. Hence, the reaction seems to undergo cyclization first before phenanthrene is finally formed via hydrogen elimination. Expanding the molecular frame of benzyl by an additional methyl group leads to the xylyl radicals and its decomposition product the xylylenes. Also important in combustion research, xylyl radicals represent the preferred decomposition products of xylene, a frequently used anti-knock agent in modern gasoline blends. After further hydrogen elimination the xylyl radicals can then form their respective xylylenes. The results of the xylyl experiments are discussed in Section 3.3. Here the gas-phase vibrational spectrum in the fingerprint region for all three isomers has been recorded for the first time in isolation. Although, all isomers have a very similar structure and symmetry, and consequently similar vibrational bands, the resolution of the experimental data was exceedingly sufficient for a clear assignment. Additionally, the dimerization products of meta- and para-xylyl could also be identified. A similar approach was taken to determine the fingerprint spectra for the xylylenes. Here, only para-xylylene could be unambiguously identified as the carrier of mass 104. For both ortho- and meta-xylylene precursors, only isomerization products were observed as the carriers of mass 104; benzocyclobutene and styrene, respectively. A possible explanation is elaborated upon in the troubleshooting Sec- tion 3.4.3.5. In the final experimental section a study on the decomposition of phthalide is presented. The objective of this experiment was mainly focused around the formation of C7 species, particularly the fulvenallenyl radical C7H5. In fact, the first experimental fingerprint spectrum of isolated C7H5 in the gas-phase was measured and is displayed in Fig. 3.45. Furthermore, the experiment demonstrates that the pyrolysis products of phthalide are excellent soot precursors, as many heavier reaction products have been identified. These include typical PAH species like naphthalene and phenanthrene as well as their methylated isomers. A large number of molecules with terminal ethynyl moieties indicate a strong influence of HACA growth in the experimental environment. However, many formation pathways of products have been discussed, which are formed involving experiment specific species, like C5H5 and C7H5, and often include expansion steps from 5- to 6-membered rings.
In this thesis the excited-state dynamics of radicals and biradicals were characterized with femtosecond pump-probe spectroscopy.
These open-shell molecules play important roles as combustion intermediates, in the formation of soot and polycyclic aromatic hydrocarbons, in atmospheric chemistry and in the formation of complex molecules in the interstellar medium and galactic clouds. In these processes molecules frequently occur in some excited state, excited either by thermal energy or radiation. Knowledge of the reactivity and dynamics of these excited states completes our understanding of these complex processes.
These highly reactive molecules were produced via pyrolysis from suitable precursors and examined in a molecular beam under collision-free conditions. A first laser now excites the molecule, and a second laser ionizes it. Time-of-flight mass spectrometry allowed a first identification of the molecule, photoelectron spectroscopy a complete characterization of the molecule - under the condition that the mass spectrum was dominated by only one mass. The photoelectron spectrum was obtained via velocity-map imaging, providing an insight in the electronic states involved. Ion velocity map imaging allowed separation of signal from direct ionization of the radical in the molecular beam and dissociative photoionization of the precursor. During this thesis a modified pBasex algorithm was developed and implemented in python, providing an image inversion tool without interpolation of data points. Especially for noisy photoelectron images this new algorithm delivers better results.
Some highlighted results:
• The 2-methylallyl radical was excited in the ππ*-state with different internal energies using three different pump wavelengths (240.6 , 238.0 and 236.0 nm). Ionized with 800 nm multi-photon probe, the photoelectron spectra shows a s-Rydberg fingerprint spectrum, a highly positive photoelectron anisotropy of 1.5 and a bi-exponential decay ( τ1= 141\pm43 fs, τ2= 4.0\pm0.2 ps for 240.6 nm pump), where the second time-constant shortens for lower wavelengths. Field-induced surface hopping dynamics calculations confirm that the initially excited ππ*-state relaxes very fast to an s-Rydberg state (first experimentally observed time-constant), and then more slowly to the first excited state/ground state (second time-constant). With higher excitation energies the conical intersection between the s-Rydberg-state and the first excited state is reached faster, resulting in shorter life-times.
• The benzyl radical was excited yith 265 nm and probed with two wavelengths, 798 nm and 398 nm. Probed with 798 nm it shows a bi-exponential decay (\tau_{1}=84\pm5 fs, \tau_{2}=1.55\pm0.12 ps), whereas with 398 nm probe only the first time-constant is observed (\tau_{1}=89\pm5 fs). The photoelectron spectra with 798 nm probe is comparable to the spectrum with 398 nm probe during the first 60 fs, at longer times an additional band appears. This band is due to a [1+3']-process, whereas with 398 nm only signal from a [1+1']-process can be observed. Non-adiabatic dynamic on the fly calculations show that the initially excited, nearly degenerate ππ/p-Rydberg-states relax very fast (first time-constant) to an s-Rydberg state. This s-Rydberg state can no longer be ionized with 398 nm, but with 798 nm ionization via intermediate resonances is still possible. The s-Rydberg state then decays to the first excited state (second time-constant), which is long-lived.
• Para-xylylene, excited with 266 nm into the S2-state and probed with 800 nm, shows a bi-exponential decay (\tau_{1}=38\pm7 fs, \tau_{2}=407\pm9 fs). The initially excited S2-state decays quickly to S1-state, which shows dissociative photoionization. The population of the S1-state is directly visible in the masses of the dissociative photoionization products, benzene and the para-xylylene -H.
• Ortho-benzyne, produced via pyrolysis from benzocyclobutendione, was excited with 266 nm in the S2 state and probed with 800 nm. In its time-resolved mass spectra the dynamic of the ortho-benzyne signal was superposed with the dynamics from dissociative photoionization of the precursor and of the ortho-benzyne-dimer. With time-resolved ion imaging gated on the ortho-benzyne these processes could be seperated, showing that the S2-state of ortho-benzyne relaxes within 50 fs to the S1-state.