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

Abstract

Lifetimes of different vibronic levels at high excess energies in the S1 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 6113 state (3290 cm-1 excess energy) at the onset of the ‘channel three’ a biexponential decay is found with a fast (τf=20 ps) and a slow (τs>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 (τf=900 fs, τs=100 ps) of the 71 state at a lower excess energy of 3077 cm-1 by a simultaneous excitation of the adjacent 61 013 0161 1 hot band leading to a vibronic state of higher excess energy and faster decay. The vibronic levels 6114 and 7111 at a higher excess energy of ≳4000 cm-1 show a biexponential decay of τf=550 fs, τs=30 ps and τf≲300 fs, τs=20 ps, respectively. The experimental results point to dynamic processes within the vibronic level manifold of the S1 state and a fast nonradiative electronic relaxation process.