Mainz 2017 – wissenschaftliches Programm
MO 8.6: Vortrag
Dienstag, 7. März 2017, 16:00–16:15, N 25
Long-lived coherence with multiple vibrational frequencies in pentafluoropyridine as a probe of ππ*- πσ* vibronic coupling — •Jonas Andreas Kus, Ole Hüter, and Friedrich Temps — Institute of Physical Chemistry, Christian-Albrechts-University Kiel, Olshausenstraße 40, D-24098 Kiel, Germany
The electronic structure of pyridine changes significantly when the hydrogen atoms are substituted by fluorine. Due to the high electronegativity of fluorine, the πσ* state is energetically lowered. This so-called perfluoro effect enables vibronic coupling of the πσ* state with the lower-lying ππ* state. Using femtosecond time-resolved time-of-flight mass spectrometry after excitation at λ = 255 nm, we observed multiple oscillations of the ion yield which can be attributed to the vibronic coupling. A pronounced oscillation at a frequency around ν = 60 cm−1 is superimposed by several oscillations of higher frequency. From results of femtosecond photoelectron imaging spectroscopy, we can rule out population transfer from the initially excited ππ* state to a different electronic state. Supported by quantum chemical calculations at the RI-SCS-CC2, CASSCF and XMCQDPT2 levels of theory, we suggest a strong distortion of the initially excited ππ* state by the coupling to the higher-lying πσ* state. The wavepacket motion in the anharmonic double well potentials modulates the ionization probability, thereby causing the observed oscillations. Taking into account our recent result for pentafluorobenze [Hüter et al., JCP 145 (2016)], strong ππ*- πσ* vibronic coupling seems to have an important general influence on the excited-state dynamics of perfluorinated aromatics.