Vibrational relaxation dynamics of the OH stretch of water at the air/water interface has been a subject of intensive research, facilitated by recent developments in ultrafast interface-selective nonlinear spectroscopy. However, a reliable determination of the vibrational relaxation dynamics in the OD stretch region at the air/D2O interface has not been yet achieved. Here, we report a study of the vibrational relaxation of the free OD carried out by time-resolved heterodyne-detected vibrational sum frequency generation spectroscopy. The results obtained with the aid of singular value decomposition analysis indicate that the vibrational relaxation (T1) time of the free OD at the air/D2O interface and air/isotopically diluted water (HOD-H2O) interfaces show no detectable isotopic dilution effect within the experimental error, as in the case of the free OH in the OH stretch region. Thus, it is concluded that the relaxation of the excited free OH/OD predominantly proceeds with their reorientation, negating a major contribution of the intramolecular energy transfer. It is also shown that the T1 time of the free OD is substantially longer than that of the free OH, further supporting the reorientation relaxation mechanism. The large difference in the T1 time between the free OD and the free OH (factor of ∼2) may indicate the nuclear quantum effect on the diffusive reorientation of the free OD/OH because this difference is significantly larger than the value expected for a classical rotational motion.

中文翻译：

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空气/水界面自由 OD 的超快振动动力学：同位素稀释效应可忽略不计，但同位素替代效应较大

空气/水界面处水的 OH 段的振动弛豫动力学一直是深入研究的主题，这得益于超快界面选择性非线性光谱学的最新发展。然而，可靠地确定 air/D 处 OD 拉伸区域的振动弛豫动力学2个O接口还没有实现。在这里，我们报告了通过时间分辨外差检测振动和频产生光谱法进行的自由 OD 振动弛豫的研究。借助奇异值分解分析得到的结果表明振动弛豫（T1个) 空气中自由 OD 的时间/D2个O界面和空气/同位素稀释水（HOD-H2个O) 界面在实验误差范围内没有显示可检测的同位素稀释效应，就像 OH 拉伸区域中游离 OH 的情况一样。因此，可以得出结论，激发的游离 OH/OD 的松弛主要随着它们的重新定向而进行，从而抵消了分子内能量转移的主要贡献。它还表明，T1个游离 OD 的时间明显长于游离 OH 的时间，进一步支持了重新定向松弛机制。T 的巨大差异1个自由 OD 和自由 OH 之间的时间（~2 的因子）可能表明核量子效应对自由 OD / OH 的扩散重定向的影响，因为这种差异明显大于经典旋转运动的预期值。