OH-stretch vibrational relaxation of HOD in liquid to superscritical D2O

The population relaxation of the OH-stretching vibration of HOD diluted in D2O is studied by time-resolved infrared (IR) pump-probe spectroscopy for temperatures of up to 700 K in the density range 12 <= p <= 58 mol/L. For selected state points of the fluid solution, transient IR spectra were recorded following resonant excitation of the v = 0 -> 1 OH stretching transition with a 200 A laser pulse centered at similar to 3500 cm(-1). Above 400 K these spectra show no indication of spectral diffusion after pump-probe delays of 0.3 ps. Over nearly the entire density range and for sufficiently high temperatures (T > 360 K), the vibrational relaxation rate constant, k(r), is strictly proportional to the dielectric constant, c, of water. Together with existing molecular dynamics simulations, this result suggests a simple linear dependence of k(r), on the number of hydrogen-bonded D2O molecules. It is shown that, for a given temperature, an isolated binary collision model is able to adequately describe the density dependence of vibrational energy relaxation even in hydrogen-bonded fluids. However, dynamic hydrogen bond breakage and formation is a source of spectral diffusion and affects the nature of the measured kr. For sufficiently high temperatures when spectral diffusion is much faster than energy transfer, the experimentally observed decays correspond to ensemble averaged population relaxation rates. In contrast, when spectral diffusion and vibrational relaxation occur on similar time scales, as is the case for ambient conditions, deviations from the linear k(r)(epsilon) relation occur because the long time decay of the v = 1 population is biased to slower relaxing HOD molecules that are only weakly connected to the hydrogen bond network.