Ultrafast internal conversion in highly excited toluene monomers and dimers

The dynamics of the internal conversion in toluene molecules and dimers exc ited to the electronic S-2 State by 150 fs laser pulses at 202 nm has been studied in pump-probe experiments by detection of the ions and the coincide nt photoelectrons. The time-dependent ion signals reflect an ultrafast (sim ilar to 50 fs) internal conversion from the excited S-2 state down to the l ower electronic S-1 and S-0 states for the monomer as well as the dimer. Th e decay of the secondarily populated S-1 state proceeds within 4.3 ps for t he toluene molecule and more than 100 ps for the dimer. The energy distribu tion of the photoelectron spectra for the monomer and the dimer ion demonst rate the significant geometry differences between the corresponding electro nic states included in the precesses of excitation, ionization and relaxati on. The structure of the photoelectron spectra measured at simultaneous abs orption of the pump and probe photons is tentatively assigned to vibrationa l modes of the ion states. The comparison of the electron spectra before an d after the internal conversion out of the S-1 state directly reflects the, growth of the vibrational energy in the secondarily populated S-1 states o f the toluene monomer and dimer.