Femtosecond pump-probe photoionization-photofragmentation spectroscopy: photoionization-induced twisting and coherent vibrational motion of azobenzene cation.
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.
Ho
Jr-Wei
JW
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China.
Chen
Wei-Kan
WK
Cheng
Po-Yuan
PY
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
D001391:Azo Compounds; C009850:azobenzene