Ho Jr-Wei, Chen Wei-Kan, Cheng Po-Yuan
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China.
J Chem Phys. 2009 Oct 7;131(13):134308. doi: 10.1063/1.3236813.
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.
我们报道了利用飞秒光电离-光解离光谱对偶氮苯阳离子超快动力学的研究。在我们的实验中，飞秒泵浦脉冲首先通过中性分子的光电离产生偶氮苯阳离子系综。随后，延迟的探测脉冲将演化中的离子系统激发到最终发生离子解离的激发态。通过监测母离子消耗或碎片离子形成随泵浦-探测延迟时间的变化来跟踪动力学过程。观察到的偶氮苯阳离子瞬态特征是，恒定的离子消耗由一个周期约为1皮秒的快速衰减振荡信号调制。理论计算表明，该振荡源于沿扭转反转坐标的振动运动，涉及CNNC中的位移和苯环扭转。振荡以约1.2皮秒的时间常数迅速衰减，这表明在这个时间尺度上发生了从活性模式到浴模式的能量耗散。
