Mechanism for vibrational relaxation in water investigated by femtosecond infrared spectroscopy

We present a study on the relaxation of the O–H stretch vibration in a dilute HDO:D₂O solution using femtosecond mid-infrared pump-probe spectroscopy. We performed one-color experiments in which the 0 ® 1 vibrational transition is probed at different frequencies, and two-color experiments in which the 1 ® 2 transition is probed. In the one-color experiments, it is observed that the relaxation is faster at the blue side than at the center of the absorption band. Furthermore, it is observed that the vibrational relaxation time T₁ shows an anomalous temperature dependence and increases from 0.74 ± 0.01 ps at 298 K to 0.90 ± 0.02 ps at 363 K. These results indicate that the O–HO.O hydrogen bond forms the dominant accepting mode in the vibrational relaxation of the O–H stretch vibration.