Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China.
J Chem Phys. 2017 Oct 28;147(16):164302. doi: 10.1063/1.5001375.
In this paper, we give a full account of our previous work [C. C. Shen et al., J. Chem. Phys. 141, 171103 (2014)] on the study of an ultrafast photoionization-induced proton transfer (PT) reaction in the phenol-ammonia (PhOH-NH) complex using ultrafast time-resolved ion photofragmentation spectroscopy implemented by the photoionization-photofragmentation pump-probe detection scheme. Neutral PhOH-NH complexes prepared in a free jet are photoionized by femtosecond 1 + 1 resonance-enhanced multiphoton ionization via the S state. The evolving cations are then probed by delayed pulses that result in ion fragmentation, and the ionic dynamics is followed by measuring the parent-ion depletion as a function of the pump-probe delay time. By comparing with systems in which PT is not feasible and the steady-state ion photofragmentation spectra, we concluded that the observed temporal evolutions of the transient ion photofragmentation spectra are consistent with an intracomplex PT reaction after photoionization from the initial non-PT to the final PT structures. Our experiments revealed that PT in [PhOH-NH] cation proceeds in two distinct steps: an initial impulsive wave-packet motion in ∼70 fs followed by a slower relaxation of about 1 ps that stabilizes the system into the final PT configuration. These results indicate that for a barrierless PT system, even though the initial PT motions are impulsive and ultrafast, the time scale to complete the reaction can be much slower and is determined by the rate of energy dissipation into other modes.
在本文中，我们全面介绍了之前的工作[C.C.Shen 等人，J.Chem.Phys.141,171103（2014）]，即在通过光致电离-光致碎裂泵浦探测方案实现的超快时间分辨离子光碎片化光谱学中，研究了苯酚-氨（PhOH-NH）复合物中的超快光致离化诱导质子转移（PT）反应。在自由喷射中制备的中性 PhOH-NH 复合物通过飞秒 1+1 共振增强多光子电离通过 S 态光致离化。然后用延迟脉冲探测演化中的阳离子，导致离子碎裂，并通过测量母体离子的耗散随泵浦-探测延迟时间的函数来跟踪离子动力学。通过与不发生 PT 的系统和稳态离子光碎片化光谱进行比较，我们得出结论，观察到的瞬态离子光碎片化光谱的时间演化与光致离化后从初始非 PT 到最终 PT 结构的内复合物 PT 反应一致。我们的实验表明，[PhOH-NH]阳离子中的 PT 分两步进行：最初的脉冲波包运动在大约 70 fs 内，然后是较慢的弛豫约 1 ps，使系统稳定在最终的 PT 结构中。这些结果表明，对于无势垒的 PT 系统，尽管初始 PT 运动是脉冲式和超快的，但完成反应的时间尺度可能要慢得多，并且取决于能量耗散到其他模式的速率。
