Long-lived coherence in pentafluorobenzene as a probe of ππ(*) - πσ(*) vibronic coupling.

The dynamics of pentafluorobenzene after femtosecond laser excitation to the optically bright ππ(*) first excited electronic state have been investigated by femtosecond time-resolved time-of-flight mass spectrometry and femtosecond time-resolved photoelectron imaging spectroscopy. The observed temporal profiles exhibit a bi-exponential decay behavior with a superimposed, long-lived, large-amplitude oscillation with a frequency of νosc = 78-74 cm(-1) and a damping time of τD = 5-2 ps. On the basis of electronic structure and quantum dynamics calculations, the oscillations have been shown to arise due to vibronic coupling between the optically bright ππ(*) state and the energetically close-lying optically dark πσ(*) state. The coupling leads to a pronounced double-well character of the lowest excited adiabatic potential energy surface along several out-of-plane modes of b1 symmetry. The optical electronic excitation initiates periodic wavepacket motion along these modes. In the out-of-plane distorted molecular configuration, the excited state acquires substantial πσ(*) character, thus modulating the ionization probability. The photoelectron spectra and the anisotropy of their angular distribution confirm the periodically changing electronic character. The ionizing probe laser pulse directly maps the coupled electron-nuclear motion into the observed signal oscillations.