Femtosecond pump-probe photoionization-photofragmentation spectroscopy of azobenzene cation

We report the studies of ultrafast dynamics of azobenzene cations using femtosecond photoionization-photofragmentation spectroscopy. In our experiment, a femtosecond pump pulse first prepares an ensemble of azobenzene cations via photoionization of neutrals. A delayed probe pulse then brings the evolving ionic system to higher states that ultimately undergo ion fragmentation. The dynamics is followed by monitoring either the parent-ion depletion or fragment-ion formation as a function of the pump-probe delay time. The observed transients for azobenzene cations are characterized by a constant ion depletion modulated by a rapidly damped oscillatory signal with a period of about 1 ps. Theoretical calculations suggest that the oscillation arises from a vibration motion along the twisting inversion coordinate involving displacements in CNNC and phenyl-ring torsions. The oscillation is damped rapidly with a time constant of about 1.2 ps, suggesting that energy dissipation from the active mode to bath modes takes place on this time scale.