Frequency dependence of the molecular reorientation of liquid water

Using multi-color femtosecond mid-infrared spectroscopy, we observe that the anisotropy dynamics of the O-D stretch vibration of HDO:H2O shows a strong dependence on the frequency of the excitation. The results indicate that a sub-ensemble of the molecules shows a fast reorientation that is accompanied by a large change of the vibrational frequency. In contrast to recent theoretical predictions [1,2], we find the probability for reorientation to be strongly frequency dependent.