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 (τ_f=20 ps) and a slow (τ_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 (τ_f=900 fs, τ_s=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 τ_f=550 fs, τ_s=30 ps and τ_f≲300 fs, τ_s=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.