Coupling between vibrational modes is essential for energy transfer and dissipation in condensed matter. For water, different O-H stretch modes are known to be very strongly coupled both within and between water molecules, leading to ultrafast dissipation and delocalization of vibrational energy. In contrast, the information on the vibrational coupling of the H-O-H bending mode of water is lacking, even though the bending mode is an essential intermediate for the energy relaxation pathway from the stretch mode to the heat bath. By combining static and femtosecond infrared, Raman, and hyper-Raman spectroscopies for isotopically diluted water with ab initio molecular dynamics simulations, we find the vibrational coupling of the bending mode differs significantly from the stretch mode: the intramode intermolecular coupling of the bending mode is very weak, in stark contrast to the stretch mode. Our results elucidate the vibrational energy transfer pathways of water. Specifically, the librational motion is essential for the vibrational energy relaxation and orientational dynamics of H-O-H bending mode.

振动模式之间的耦合对于凝结物中的能量传递和耗散至关重要。对于水，已知不同的OH拉伸模式在水分子内部和之间非常强地耦合，从而导致振动能量的超快耗散和离域。相反，即使弯曲模式是从拉伸模式到热浴的能量松弛路径的必不可少的中间部分，也缺少关于水的HOH弯曲模式的振动耦合的信息。通过将同位素稀释水的静态和飞秒红外光谱，拉曼光谱和超拉曼光谱与从头算分子动力学模拟相结合，我们发现弯曲模式的振动耦合与拉伸模式有很大不同：与拉伸模式形成鲜明对比的是，弯曲模式的模式内分子间耦合非常弱。我们的结果阐明了水的振动能量传递途径。具体而言，自由运动对于HOH弯曲模式的振动能松弛和定向动力学至关重要。