@unpublished{HocheSchmittHumeniuketal.2017,
  author    = {Hoche, Joscha and Schmitt, Hans-Christian and Humeniuk, Alexander and Fischer, Ingo and Mitrić, Roland and R{\"o}hr, Merle I. S.},
  title     = {The mechanism of excimer formation: an experimental and theoretical study on the pyrene dimer},
  series = {Physical Chemistry Chemical Physics},
  journal   = {Physical Chemistry Chemical Physics},
  doi       = {10.1039/C7CP03990E},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-159656},
  year      = {2017},
  abstract  = {The understanding of excimer formation in organic materials is of fundamental importance, since excimers profoundly influence their functional performance in applications such as light-harvesting, photovoltaics or organic electronics. We present a joint experimental and theoretical study of the ultrafast dynamics of excimer formation in the pyrene dimer in a supersonic jet, which is the archetype of an excimer forming system. We perform simulations of the nonadiabatic photodynamics in the frame of TDDFT that reveal two distinct excimer formation pathways in the gas-phase dimer. The first pathway involves local excited state relaxation close to the initial Franck-Condon geometry that is characterized by a strong excitation of the stacking coordinate exhibiting damped oscillations with a period of 350 fs that persist for several picoseconds. The second excimer forming pathway involves large amplitude oscillations along the parallel shift coordinate with a period of ≈900 fs that after intramolecular vibrational energy redistribution leads to the formation of a perfectly stacked dimer. The electronic relaxation within the excitonic manifold is mediated by the presence of intermolecular conical intersections formed between fully delocalized excitonic states. Such conical intersections may generally arise in stacked π-conjugated aggregates due to the interplay between the long-range and short-range electronic coupling. The simulations are supported by picosecond photoionization experiments in a supersonic jet that provide a time-constant for the excimer formation of around 6-7 ps, in good agreement with theory. Finally, in order to explore how the crystal environment influences the excimer formation dynamics we perform large scale QM/MM nonadiabatic dynamics simulations on a pyrene crystal in the framework of the long-range corrected tight-binding TDDFT. In contrast to the isolated dimer, the excimer formation in the crystal follows a single reaction pathway in which the initially excited parallel slip motion is strongly damped by the interaction with the surrounding molecules leading to the slow excimer stabilization on a picosecond time scale.},
  language  = {en}
}
@unpublished{RoederHumeniukGiegerichetal.2017,
  author    = {R{\"o}der, Anja and Humeniuk, Alexander and Giegerich, Jens and Fischer, Ingo and Poisson, Lionel and Mitric, Roland},
  title     = {Femtosecond Time-Resolved Photoelectron Spectroscopy of the Benzyl Radical},
  series = {Physical Chemistry Chemical Physics},
  journal   = {Physical Chemistry Chemical Physics},
  doi       = {10.1039/C7CP01437F},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-159474},
  year      = {2017},
  abstract  = {We present a joint experimental and computational study of the nonradiative deactivation of the benzyl radical, C\(_7\)H\(_7\) after UV excitation. Femtosecond time-resolved photoelectron imaging was applied to investigate the photodynamics of the radical. The experiments were accompanied by excited state dynamics simulations using surface hopping. Benzyl has been excited at 265 nm into the D-band (\(\pi\pi^*\)) and the dynamics was probed using probe wavelengths of 398 nm or 798 nm. With 398 nm probe a single time constant of around 70-80 fs was observed. When the dynamics was probed at 798 nm, a second time constant \(\tau_2\)=1.5 ps was visible. It is assigned to further non-radiative deactivation to the lower-lying D\(_1\)/D\(_2\) states.},
  language  = {en}
}