OH-Stretch Vibrational Relaxation of HOD in Liquid to Supercritical D₂O

Abstract

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 <or= rho <or= 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 fs laser pulse centered at approximately 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, kr, is strictly proportional to the dielectric constant, epsilon, of water. Together with existing molecular dynamics simulations, this result suggests a simple linear dependence of kr 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 kr(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.

OH and NH Stretching Vibrational Relaxation of Liquid Ethanolamine

“…This behavior is even more surprising in light of the finding that the pure density dependence of the isothermal VER rate is exceptionally well reproduced by a simple description invoking isolated binary collisions (IBC) between attractive hard sphere molecules [18]. Such a model would in turn predict an increase of the VER rate with temperature as the average particle velocities and hence, the collision frequencies increase.…”

“…Finally, a systematic study of the temperature and density dependence of the VER dynamics from liquid to supercritical water reveals a fascinating new physicochemical anomaly, namely an increase of the VER rate with decreasing temperature [17,18]. This behavior is even more surprising in light of the finding that the pure density dependence of the isothermal VER rate is exceptionally well reproduced by a simple description invoking isolated binary collisions (IBC) between attractive hard sphere molecules [18].…”

“…This additional complexity can be lifted by studying the monodeuterated isotopomeric systems HOD dissolved in H 2 O or in D 2 O, respectively [13][14][15][16][17][18][19]. Furthermore, in these systems the OH (or OD) vibrator finds itself highly diluted in a bath consisting exclusively of OD (or OH) oscillators.…”

“…This deuteration also circumvents a canonical heating of the sample that will inevitably occur upon laser induced vibrational excitation of the neat liquid. Depopulation of the v = 1 vibrational state occurs on a time scale of about 1 ps and is believed to occur through the bending manifold [5,14,18,[20][21][22][23][24].…”

Two-Dimensional Infrared Spectroscopy of Supercooled Water

“…19,20 From temperature dependent simulations 21,22 and experimental studies of isotope diluted water, ranging from ambient conditions 23,24 up to the supercritical regime 25 is understood that FFCF decay depends critically on temperature. Furthermore from time resolved optical Kerr effect (HD-OKE) measurements in supercooled water has been observed that water dynamics are characterized by a distribution of different timescales due to the presence of a variety of relaxing structures.…”