H₂O Condensation Coefficient and Refractive Index for Vapor-Deposited Ice from Molecular Beam and Optical Interference Measurements

Abstract

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°) and beam energy (1−40 kcal/mol). The condensation coefficient, α, was dependent on both the incident H2O flux and the desorption H2O flux at the various surface temperatures. The magnitude of α varied continuously from unity at T < 130 K to zero at higher temperatures. The optical interference experiments yielded condensation coefficients and sticking coefficients of α = 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, ρ, 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 ρ = 0.82 g/cm3 at 90 K to ρ = 0.93 g/cm3 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.

The vapor pressure over nano-crystalline ice

“…Sack and Baragiola (1993) do not discuss the temperature uncertainty of their sample and the accuracy of their measurements in detail. Comparing the results of Brown et al (1996) and Sack and Baragiola (1993) to the results of the MICE-TRAPS experiments below 160 K, we assume that the reported desorption rates in both publications are rather accurate and support our measurements of an elevated vapor pressure with respect to ice I h between a factor of 2 and 3.…”

“…The authors of both articles do not discuss potential causes of the measured elevated vapor pressure with respect to ice I h . However, in the case of Brown et al (1996) a temperature error of 2 K is assumed, which is large enough to make their results agree with the vapor pressure of ice I h within the limits of error. Sack and Baragiola (1993) do not discuss the temperature uncertainty of their sample and the accuracy of their measurements in detail.…”

“…These measurements typically employed a quadrupole mass spectrometer (QMS) and/or a quartz crystal microbalance to measure desorption rates. Desorption rates can be used to infer saturation vapor pressures under the well-supported assumption that the sticking coefficient for water molecules on water ice is unity at these temperatures (Batista et al, 2005;Brown et al, 1996;Gibson et al, 2011;Kong et al, 2014). Measuring water vapor desorption rates at the temperatures under investigation is a challenging task and previous experiments were influenced by contamination issues, showed a very large degree of scattering in the data or yielded unphysically low vapor pressures below that of ice I h (Bryson et al, 1974;Fraser et al, 2001;La Spisa et al, 2001).…”

“…2. Brown et al measured temperature-dependent desorption rates with a QMS (Brown et al, 1996). We converted their parameterized data to normalized vapor pressure values and show them as a black dashed line in Fig.…”

“…At temperatures below 160 K, however, only a limited number of desorption rate measurements of ice crystallized from ASW using quadrupole mass spectrometers and quartz crystal microbalances is available that may be used to calculate the saturation vapor pressure over the ice phase (Brown et al, 1996;Bryson et al, 1974;Fraser et al, 2001;La Spisa et al, 2001;Sack and Baragiola, 1993;Smith et al, 2011;Speedy et al, 1996).…”