Ultrafast Internal Conversion in Highly Excited Toluene Monomers and Dimers^†

The dynamics of the internal conversion in toluene molecules and dimers excited to the electronic S₂ state by 150 fs laser pulses at 202 nm has been studied in pump−probe experiments by detection of the ions and the coincident photoelectrons. The time-dependent ion signals reflect an ultrafast (50 fs) internal conversion from the excited S₂ state down to the lower electronic S₁ and S₀ states for the monomer as well as the dimer. The decay of the secondarily populated S₁ state proceeds within 4.3 ps for the toluene molecule and more than 100 ps for the dimer. The energy distribution of the photoelectron spectra for the monomer and the dimer ion demonstrate the significant geometry differences between the corresponding electronic states included in the processes of excitation, ionization and relaxation. The structure of the photoelectron spectra measured at simultaneous absorption of the pump and probe photons is tentatively assigned to vibrational modes of the ion states. The comparison of the electron spectra before and after the internal conversion out of the S₂ state directly reflects the growth of the vibrational energy in the secondarily populated S₁ states of the toluene monomer and dimer.