Using femtosecond two-color infrared pump-probe spectroscopy, we study the vibrational relaxation of the O-H stretch vibrations of liquid H2O after excitation of the overtone transition. The overtone transition has its maximum at 6900 cm(-1) (1.45 mu m), which is a relatively high frequency in view of the central frequency of 3400 cm(-1) of the fundamental transition. The excitation of the overtone leads to a transient induced absorption of two-exciton states of the O-H stretch vibrations. When the overtone excitation frequency is tuned from 6600 to 7200 cm(-1), the vibrational relaxation time constant of the two-exciton states increases from 400 +/- 50 fs to 540 +/- 40 fs. These values define a limited range of relatively long relaxation time constants compared to the range of relaxation time constants of 250-550 fs that we recently observed for the one-exciton O-H stretch vibrational state of liquid H2O (S. T. van der Post et al., Nature Comm. 2015, 6, 8384). We explain the high central frequency and the limited range of relatively long relaxation time constants of the overtone transition from the destructive quantum interference of the mechanical and electrical anharmonic contributions to the overtone transition probability. As a result of this destructive interference, the overtone transition of liquid H2O is dominated by molecules of which the O-H groups donate relatively weak hydrogen bonds to other H2O molecules.
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机译：使用飞秒二色红外泵浦探针光谱法，研究了泛音跃迁激发后液态H2O的O-H拉伸振动的振动弛豫。泛音过渡的最大值为6900 cm（-1）（1.45μm），鉴于基本过渡的中心频率为3400 cm（-1），这是一个相对较高的频率。泛音的激发导致O-H拉伸振动的两个激子状态的瞬态感应吸收。当泛音激励频率从6600调整到7200 cm（-1）时，两个激子态的振动弛豫时间常数从400 +/- 50 fs增加到540 +/- 40 fs。与我们最近在液态H2O的一激子OH拉伸振动状态下观察到的250-550 fs的弛豫时间常数范围相比，这些值限定了相对较长的弛豫时间常数的有限范围（ST van der Post等， Nature Comm。2015，6，8384）。我们解释了高中心频率和泛音过渡相对较长的弛豫时间常数的有限范围，这是由于机械和电气非谐谐波的破坏性量子干扰对泛音过渡概率造成的。这种破坏性干扰的结果是，液态H2O的泛音跃迁受其O-H基团向其他H2O分子提供相对弱的氢键的分子支配。
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