Subpicosecond spectroscopy of liquid water in the infrared: Effect of deuteration on the structural and vibrational dynamics

The spectral substructure and the dynamics of the OH- and OD-stretching bands of HDO in the diluted isotopic mixtures with D₂O and for the first time H₂O are investigated using 2-color IR spectroscopy with tunable subpicosecond pulses. From the observed transient band shapes, we derive four major spectral components within the OH-band of HDO in the solvent D₂O peaked at approximately 3330 cm^-1 (I), 3390 cm^-1(II), 3460 cm^-1 (III), and 3520 cm^-1 (IV), as well as three corresponding species in the case of the OD-band in the solvent H₂O. The components display no distinct temperature dependence of position and spectral width in the investigated temperature range 273-343 K. Comparing the OH-stretching mode of HDO:D₂O to the OD-vibration of HDO:H₂O, the population lifetime increases significantly from 1.0 ps for OH to 1.8 ps (±0.2 ps) for OD at room temperature, whereas spectral relaxation, assigned to structural changes, seems to slow from 1.0 ± 0.4 ps to 2.0 ± 0.8 ps in the protonated water environment.