H2O condensation coefficient and refractive index for vapor-deposited ice from molecular beam and optical interference measurements

The condensation of H2O on ice multilayers on Ru(001) was studied using both molecular beam and optical interference techniques as a function of surface temperature. From the beam reflection technique, the H2O sticking coefficient, S, was determined to be S = 0.99 +/- 0.03 at temperatures between 85 and 150 K and was independent of incident angle (0-70 degrees) and beam energy (1-40 kcal/mol). Thf condensation coefficient, alpha, was dependent on both the incident H2O flux and the desorption H2O flux at the various surface temperatures. The magnitude of alpha varied continuously from unity at T < 130 K to zero at higher temperatures. The optical interference experiments yielded condensation coefficients and sticking coefficients of alpha = S = 0.97 +/- 0.10 at temperatures from 97 to 145 K where the H2O desorption flux was negligible with respect to the incident flux. The optical interference measurements monitored the ice film thickness versus H2O exposure time and were dependent on the refractive index, n, and the density, rho, of the vapor-deposited ice. Consequently, the combined molecular beam and optical interference measurements provided a means to evaluate the refractive index and density for vapor-deposited ice as a function of surface temperature. The values of the refractive index varied from n = 1.27 at 90 K to n = 1.31 at 130 K. The calculated densities varied from rho = 0.82 g/cm(3) at 90 K to rho = 0.93 g/cm(3) at 130 K. Previous optical interference data were also reanalyzed to yield refractive indices and ice densities for films grown at surface temperatures between 20 and 150 K. Both the refractive index and density increased monotonically with increasing growth temperature. The lower refractive index and density at lower temperatures indicate that microporous ice films are formed when H2O deposits on substrates at T < 120 K.