title: Femtosecond pump-probe photoionization-photofragmentation spectroscopy: photoionization-induced twisting and coherent vibrational motion of azobenzene cation.
abstract: We report studies of ultrafast dynamics of azobenzene cation using femtosecond photoionization-photofragmentation spectroscopy. In our experiments, a femtosecond pump pulse first produces an ensemble of azobenzene cations via photoionization of the neutrals. A delayed probe pulse then brings the evolving ionic system to excited 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 cation 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 in this time scale.
authors.affiliation: Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China.
authors.forename: Jr-Wei
authors.initials: JW
authors.lastname: Ho
authors.forename: Wei-Kan
authors.initials: WK
authors.lastname: Chen
authors.forename: Po-Yuan
authors.initials: PY
authors.lastname: Cheng
mesh_terms: D016880:Anisotropy; D001391:Azo Compounds / Q000737:chemistry*; D004583:Electrons; D007700:Kinetics; D008958:Models, Molecular; D008968:Molecular Conformation; D009038:Motion; D055668:Photochemical Processes; D013057:Spectrum Analysis; D013237:Stereoisomerism; D014732:Vibration
chemical_list: D001391:Azo Compounds; C009850:azobenzene