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. 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 originally 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 Rapid damped oscillatory signal with a period of about 1 ps. 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.