Department of Chemistry, School of Science, Tokyo Institute of Technology, Meguro 152-8550, Japan.
Department of Chemistry, School of Science, Kitasato University, 1-15-1 Kitazato, Minami, Sagamihara, Kanagawa 252-0373, Japan.
J Phys Chem A. 2023 Jun 15;127(23):4964-4978. doi: 10.1021/acs.jpca.3c02138. Epub 2023 May 31.
We report the creation and observation of vibrational wave packets pertinent to torsional motion in a biphenyl derivative in its electronic ground-state manifold. Adiabatically cooled molecular samples of 2-fluorobiphenyl were irradiated by intense nonresonant ultrashort laser pulses to drive impulsive stimulated Raman excitation of torsional motion. Spectral change due to the nonadiabatic vibrational excitation is probed in a state-selective manner using resonance-enhanced two-photon ionization through the S ← S electronic transition. The coherent nature of the excitation was exemplified by adopting irradiation with a pair of pump pulses: observed signals for excited torsional levels exhibit oscillatory variations against the mutual delay between the pump pulses due to wave-packet interference. By taking the Fourier transform of the time course of the signals, energy intervals among torsional levels with = 0-3 were determined and utilized to calibrate a density functional theory (DFT)-calculated torsional potential-energy function. Time variation of populations in the excited torsional levels was assessed experimentally by measuring integrated intensities of the corresponding transitions while scanning the delay. Early time enhancement of the population (up to ∼2 ps) and gradual degradation of coherence (within ∼20 ps) appears. To explain the observed distinctive features, we developed a four-dimensional (4D) dynamical calculation in which one-dimensional (1D) quantum-mechanical propagation of the torsional motion was followed by solving the time-dependent Schrödinger equation, whereas three-dimensional (3D) molecular rotation was tracked by classical trajectory calculations. This hybrid approach enabled us to reproduce experimental results at a reasonable computational cost and provided a deeper insight into rotational effects on vibrational wave-packet dynamics.
我们报告了在联苯衍生物的电子基态中与扭转运动相关的振动波包的产生和观测。通过强非共振超短激光脉冲，对 2-氟联苯的绝热冷却分子样品进行辐照，以驱动扭转运动的脉冲受激拉曼激发。通过通过 S ← S 电子跃迁进行共振增强双光子电离，以选择性的方式探测由于非绝热振动激发引起的光谱变化。采用一对泵浦脉冲进行辐照，证明了激发的相干性质：观察到的激发扭转能级信号由于波包干涉而表现出与泵浦脉冲之间的相互延迟相对应的振荡变化。通过对信号的时间历程进行傅里叶变换，确定了扭转能级之间的能量间隔， = 0-3，并利用它们来校准密度泛函理论（DFT）计算的扭转势能函数。通过在扫描延迟时测量相应跃迁的积分强度，实验评估了激发扭转能级中布居的时间变化。观察到早期时间的布居增强（高达 ∼2 ps）和相干性的逐渐退化（在 ∼20 ps 内）。为了解释观察到的独特特征，我们开发了一个四维度（4D）动力学计算，其中通过求解含时薛定谔方程，对扭转运动的一维量子力学传播进行了追踪，而三维（3D）分子旋转则通过经典轨迹计算进行了追踪。这种混合方法使我们能够以合理的计算成本重现实验结果，并深入了解旋转对振动波包动力学的影响。
