Fecko Christopher J, Loparo Joseph J, Roberts Sean T, Tokmakoff Andrei
Department of Chemistry and George R. Harrison Spectroscopy Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
J Chem Phys. 2005 Feb 1;122(5):54506. doi: 10.1063/1.1839179.
We present an investigation into hydrogen bonding dynamics and kinetics in water using femtosecond infrared spectroscopy of the OH stretching vibration of HOD in D(2)O. Infrared vibrational echo peak shift and polarization-selective pump-probe experiments were performed with mid-IR pulses short enough to capture all relevant dynamical processes. The experiments are self-consistently analyzed with a nonlinear response function expressed in terms of three dynamical parameters for the OH stretching vibration: the frequency correlation function, the lifetime, and the second Legendre polynomial dipole reorientation correlation function. It also accounts for vibrational-relaxation-induced excitation of intermolecular motion that appears as heating. The long time, picosecond behavior is consistent with previous work, but new dynamics are revealed on the sub-200 fs time scale. The frequency correlation function is characterized by a 50 fs decay and 180 fs beat associated with underdamped intermolecular vibrations of hydrogen bonding partners prior to 1.4 ps exponential relaxation. The reorientational correlation function observes a 50 fs librational decay prior to 3 ps diffusive reorientation. Both of these correlation functions compare favorably with the predictions from classical molecular dynamics simulations. The time-dependent behavior can be separated into short and long time scales by the 340 fs correlation time for OH frequency shifts. The fast time scales arise from dynamics that are mainly local: fluctuations in hydrogen bond distances and angles within relatively fixed intermolecular configurations. On time scales longer than the correlation time, dephasing and reorientations reflect collective reorganization of the liquid structure. Since the OH transition frequency and dipole are only weakly sensitive to these collective coordinates, this is a kinetic regime which gives an effective rate for exchange of intermolecular structures.
我们利用D₂O中HOD的OH伸缩振动的飞秒红外光谱，对水中的氢键动力学和动力学进行了研究。使用足够短的中红外脉冲进行红外振动回波峰移和偏振选择性泵浦 - 探测实验，以捕捉所有相关的动力学过程。用一个非线性响应函数对实验进行自洽分析，该函数用OH伸缩振动的三个动力学参数表示：频率相关函数、寿命和二阶勒让德多项式偶极重取向相关函数。它还考虑了作为加热出现的分子间运动的振动弛豫诱导激发。长时间的皮秒行为与先前的工作一致，但在低于200飞秒的时间尺度上揭示了新的动力学。频率相关函数的特征是在1.4皮秒指数弛豫之前，与氢键伙伴的欠阻尼分子间振动相关的50飞秒衰减和180飞秒拍频。重取向相关函数在3皮秒扩散重取向之前观察到50飞秒的摆动衰减。这两个相关函数与经典分子动力学模拟的预测结果都比较吻合。OH频移的340飞秒相关时间可将时间相关行为分为短时间和长时间尺度。快速时间尺度源于主要是局部的动力学：相对固定的分子间构型内氢键距离和角度的波动。在比相关时间更长的时间尺度上，去相位和重取向反映了液体结构的集体重组。由于OH跃迁频率和偶极仅对这些集体坐标弱敏感，这是一个动力学区域，给出了分子间结构交换的有效速率。
