Lifetime of the benzene dimer in the S2 electronic state

The lifetime of the benzene dimer in the S2 state has been determined directly by pump-probe measurements with 160 fs laser pulses. The value of ≈ 40 fs obtained characterises the fast internal conversion to the S0 as well as to the S1 state. The probability of the S2 to S1 state conversion for the dimer is small but significant and a factor of about 2 larger than that for the benzene monomer. These results explain in a direct manner corresponding experiments with ns laser pulses where two-photon enhancement of the ion yield is observed only when the second photon has sufficient energy to ionise the S1 state.