Fast decay of high vibronic S1 states in gas-phase benzene

Lifetimes of different vibronic levels at high excess energies in the S-1 state of isolated benzene molecules are measured using for the first time a two-photon ionization pump-probe technique with UV femtosecond pulses. For the 6(1)1(3) State (3290cm(-1) excess energy) at the onset of the 'channel three' a biexponential decay is found with a fast (tau(f) = 20 ps) and a slow (tau(s) > 500 ps) component. The values are in line with previous sub-Doppler high-resolution measurements of this band in our laboratory. We explain the measured faster decay (tau(f) = 900fs, tau(s) = 100ps) of the 7(1) state at a lower excess energy of 3077 cm(-1) by a simultaneous excitation of the adjacent 6(0)(1)1(0)(3)16(1)(1) hot band leading to a vibronic state of higher excess energy and faster decay. The vibronic levels 6(1)1(4) and 7(1)1(1) at a higher excess energy of greater than or similar to 4000 cm(-1) show a biexponential decay of tau(f) = 550 fs, tau(s) = 30 ps and tau(f) less than or similar to 300 fs, tau(s) = 20 ps, respectively. The experimental results point to dynamic processes within the vibronic level manifold of the S-1 state and a fast nonradiative electronic relaxation process.