OD stretch vibrational relaxation of HOD in liquid to supercritical H2O

The population relaxation of the OD stretching vibration of HOD diluted in H2O is studied by time-resolved infrared pump-probe spectroscopy for temperatures between 278 and 663 K in the density range 0.28 <=rho <= 1.01 g/cm(3). Transient spectra recorded after exciting the nu=0 -> 1 OD stretching transition at low temperatures show a delay between excited state decay and formation of the thermalized spectrum pointing to an intermediately populated state. Above 400 K, the rates of excited state decay and ground state recovery become equivalent and the intermediate state is not detectable anymore. Over the entire thermodynamic range, the derived OD stretch relaxation rate constant k(r) depends linearly on the static dielectric constant epsilon of water, indicating a correlation of k(r) with the average hydrogen bond connectivity of HOD within the H2O network. However, in contrast to the OH stretch relaxation rate constant of the complementary system of HOD in D2O, the low density data of k(r)(epsilon) extrapolate to a nonzero intercept for epsilon -> 1. Our analysis suggests that at ambient conditions the OD excited state is mainly depopulated by a direct nu=1 -> 0 transition, avoiding the excited nu=1 HOD bending state. Therefore, at room temperature the detected intermediate is assigned to a nonthermalized state with respect to nuclear degrees of freedom of the solvent molecules, and subsequent formation of the final product spectrum is related to a rearrangement of the hydrogen bond network. Passing over to the gas phase the excited OD stretch state shifts into close resonance with the HOD bend overtone, thereby opening up an additional relaxation channel. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3151673]