We report the results of a fast thermal desorption spectroscopy study of the H/D isotopic exchange kinetics in a few micrometer thick, pure polycrystalline ice film and in ice films doped with HCl. Using the isotopic exchange reaction as a probe of transport processes in ice, we determined the effective H/D interdiffusion coefficients, D(eff), in pure and doped polycrystalline ice at temperatures ranging from -18 to -1 degree C. In the case of pure polycrystalline ice, D(eff) demonstrates an Arrhenius dependence on temperature with an effective activation energy of 69+/-3 kJ mol(-1) and a pre-exponential of 10(9+/-0.5) microm(2) ms(-1) up to -2 degrees C. According to our analysis, H/D interdiffusion coefficient at the grain boundaries also shows an Arrhenius dependence on temperature with an activation energy of 69+/-3 kJ mol(-1) and a pre-exponential of 10(11+/-1) microm(2) ms(-1). However, the addition of 0.04% of HCl results in a marked deviation of D(eff) from Arrhenius law at -8 degrees C, which is attributed to premelting at intersections of grain boundaries. We discuss the structure and transport properties of condensed aqueous phase at grain boundaries in polycrystalline ice at various temperatures.

Fast thermal desorption spectroscopy was used to investigate the vaporization kinetics of thin (50-100 nm) H(2)O(18) and HDO tracer layers from 2-5 microm thick polycrystalline H(2)O(16) ice films at temperatures ranging from -15 to -2 degrees C. The isothermal desorption spectra of tracer species demonstrate two distinct peaks, alpha and beta, which we attribute to the vaporization of H(2)O(18) initially trapped at or near the grain boundaries and in the crystallites of the polycrystalline ice, respectively. We show that the diffusive transport of the H(2)O(18) and HDO tracer molecules in the bulk of the H(2)O(16) film is slow as compared to the film vaporization. Thus, the two peaks in the isothermal spectra are due to unequal vaporization rates of H(2)O(18) from grain boundary grooves and from the crystallites and, therefore, can be used to determine independently the vaporization rate of the single crystal part of the film and rate of thermal etching of the film. Our analysis of the tracer vaporization kinetics demonstrates that the vaporization coefficient of single crystal ice is significantly greater than those predicted by the classical vaporization mechanism at temperatures near ice melting point. We discuss surface morphological dynamics and the bulk transport phenomena in single crystal and polycrystalline ice near 0 degrees C.

Using fast thermal desorption spectroscopy, a novel technique developed in our laboratory, we investigated the kinetics of HD isotopic exchange in 3 microm thick polycrystalline H2O ice films containing D2O layers at thicknesses ranging from 10 to 300 nm at a temperature of -2.0+/-1.5 degrees C. According to our results over the duration of a typical fast thermal desorption experiment (3-4 ms), the isotopic exchange is confined to a 50+/-10 nm wide reaction zone located at the boundary between polycrystalline H2O and D2O ice. Combining these data with a theoretical analysis of the diffusion in polycrystalline medium, we establish the range of possible values for water self-diffusion coefficients and the grain boundary widths characteristic of our ice samples. Our analysis shows that for the grain boundary width on the order of a few nanometers, the diffusivity of D2O along the grain boundaries must be at least two orders of magnitude lower than that in bulk water at the same temperature. Based on these results, we argue that, in the limit of low concentrations of impurities, polycrystalline ice does not undergo grain boundary premelting at temperatures up to -2 degrees C.

No abstract

We report results of ultrafast scanning calorimetry (USC) measurements of enthalpy relaxation time for propanol, toluene, pentanol, decalin, and 2-ethyl-1-hexanol in a temperature range from 120to180K. These new data show that the enthalpy relaxation times measured in USC experiments are within one order of magnitude of those derived from dielectric spectroscopy studies. Thus, we demonstrate that Johari’s critique of the USC study of pure and doped amorphous solid water is without merit, and that USC experiments do provide evidence against assignment of the glass transition temperature of amorphous solid water to 136K.