Femtosecond Relaxation Dynamics of Molecular Rydberg States Using Time-Resolved Multiphoton Ionization

Lifetimes for 3Rg Rydberg states of benzene and perdeuterobenzene, and for $4d$ Rydberg states of toluene and perdeuterotoluene, have been measured by time-resolved ionization following two-photon excitation with 0.19-ps laser pulses. For benzene, this lifetime is 70±{}20 fs. This is the shortest molecular process ever resolved by direct measurement. For toluene, the lifetime is 170±{}20 fs. The decay mechanism is not established, but could involve internal conversion to high vibrational levels of the ground state.