Ultrafast nonradiative dynamics in electronically excited hexafluorobenzene by femtosecond time-resolved mass spectrometry.

The fast nonradiative decay dynamics of the lowest two excited pipi() electronic states (S(2) and S(3)) of hexafluorobenzene have been investigated by using femtosecond time-resolved time-of-flight mass spectrometry. The molecules were excited at wavelengths between 265 nm > or = lambda(pump) > or = 217 nm and probed by four- and three-photon ionization at lambda(probe)=775 nm. The observed temporal profiles exhibit two exponential decay times (tau(1)=0.54-0.1 ps and tau(2)=493-4.67 ps, depending on the excitation wavelength) and a superimposed coherent oscillation with vibrational frequency nu(osc)=97 cm(-1) and damping time tau(D) that is two to three times longer than the respective tau(1). The first decay component (tau(1)) is assigned to rapid radiationless transfer from the excited optically bright pipi() electronic state (S(2) or S(3), respectively) through a conical intersection (CI) to the lower-lying optically dark pisigma() state (S(1)) of the molecule; the second component (tau(2)) is attributed to the subsequent slower relaxation from the S(1) state back to the electronic ground state (S(0)). tau(2) dramatically decreases with increasing vibronic excitation energy up to the CI connecting the pisigma() with the S(0) state. The coherent oscillation is identified as nuclear motion along the out-of-plane vibration nu(16a) (notation as for benzene), which has e(2u) symmetry and acts as coupling mode between the pipi() and pisigma() states.