Highly Selective Relaxation of the OH Stretching Overtones in Isolated HDO Molecules Observed by Infrared Pump–Repump–Probe Spectroscopy

A quantitative investigation of the relaxation dynamics of higher-lying vibrational states is afforded by a novel method of infrared pump–repump–probe spectroscopy. The technique is used to study the dynamics of OH stretching overtones in NaClO4·HDO monohydrate. We observe a continuous decrease of the energy separation for the first four states, i.e. v01 = 3575 cm–1, v12 = 3370 cm–1, and v23 = 3170 cm–1, respectively. The population lifetime of the first excited state is 7.2 ps, while the one of the second excited state is largely reduced to 1.4 ps. The relaxation of the v = 2 state proceeds nearly quantitatively to the v = 1 state. The new information on the OH stretching overtones demands improved theoretical potentials and modeling of the H bond interactions. This work shows the potential of the new technique for the precise study of complex vibrational relaxation pathways.