Temperature dependence of vibrational relaxation of the OH bending excitation in liquid H2O

Mid-infrared pump–probe spectroscopy measurements of the OH bending vibration in pure liquid water were performed at different temperatures. The population lifetime increases from 170 ± 15 fs at T = 295 K to 250 ± 15 fs at T = 348 K. This temperature dependence can be explained by the decrease in the spectral overlap between the OH bending and the high-frequency librational modes, supporting the relaxation scenario that the bend vibrational energy is dominantly transferred to librational motions. It was also found that the temperature dependence becomes absent below room temperature, the possible origin for which is discussed.