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Using the pump–probe technique with 130 fs laser pulses near 200 nm and near 266 nm the internal conversion of the pyrazine molecule excited to the S2 state has been studied. The lifetime of the S2 state due to internal conversion to the lower electronic states is τIC(2)=(20±10) fs while the lifetime of the secondarily populated S1 state is τIC(1)=(22±1) ps. The results of femtosecond time-resolved electron spectroscopy directly demonstrate the variation of the electron configuration during the internal conversion: The electron spectrum changes significantly on the fs time scale for pyrazine ions produced by ionization via the S2 state with ππ* character and by ionization of S1 state molecules with nπ* configuration after the internal conversion, respectively. The results obtained confirm theoretical estimations of Domcke and co-workers [J. Chem. Phys. 95, 7806 (1991); J. Phys. Chem. 97, 12466 (1993)] who describe the internal conversion in the pyrazine molecule on the basis of a conical intersection of the corresponding potential energy surfaces.

The dynamics of several prototypical molecular systems after excitation with femtosecond laser pulses at 155 nm has been studied in pump-probe experiments. The vacuum ultraviolet ͑VUV͒ pump pulses with a pulse width of 350-450 fs were generated by near-resonant four-wave difference frequency mixing in argon. The careful analysis of the time-dependent ion signals has allowed us to determine the lifetime of the excited molecular states down to about 30 fs. The extremely short lifetime of water molecules excited to the repulsive à state has been directly observed for the first time: D р20 fs. For molecular oxygen highly excited in the Schumann-Runge band, a decay time of 40Ϯ20 fs was obtained. The lifetimes of ethylene and chloroethylenes as well as of benzene and toluene reaching from 40 up to 180 fs are primarily caused by internal conversion. The decay times D ϭ(1.9Ϯ0.1) and D ϭ(90Ϯ20) ps obtained for carbon disulfide and nitric oxide, respectively, are due to predissociation of the VUV excited states.

The dynamics of the internal conversion in toluene molecules and dimers excited 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 coincident photoelectrons. The time-dependent ion signals reflect an ultrafast (∼50 fs) internal conversion from the excited S 2 state down to the lower electronic S 1 and S 0 states for the monomer as well as the dimer. The decay of the secondarily populated S 1 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 S 2 state directly reflects the growth of the vibrational energy in the secondarily populated S 1 states of the toluene monomer and dimer.

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