adsorption by ASW films grown with 0 5 20' Bruce D. Kay? was similar to adsorption by a cwstalline ice film. However, for 0 > 30°, the amount of N,The morphology of amorphous solid water grown by vapor deposition was adsorbed by the ASW increased markedly, found t o depend strongly on the angular distribution of the water molecules reaching a maximum near 0 = 70'. At the incident from the gas phase. sternati tic variation of the incident angle during maximum, the ASW films adsorbed more than deposition using a collimated beam of water led to the growth of nonporous 20 times the amount of N, adsorbed by a t o highly porous amorphous solid water. The physical and chemical properties

The adsorption of N2 was used to investigate the porosity/morphology of thin films of amorphous solid water. Molecular beams were used to vapor deposit amorphous solid water films on a Pt(111) crystal at a variety of incident growth angles. The amount of N2 adsorbed by the amorphous solid water depends very sensitively on the growth angle and thermal history of the film. For normal and nearly normal incidence growth, the water films are relatively dense and smooth and adsorb only a small amount of N2. For larger growth angles, the films are porous and adsorb large quantities of N2 with apparent surface areas as high as ∼2700 m2/g. The physical and chemical properties of amorphous solid water are of interest because of its presence in astrophysical environments. The observations have important implications for laboratory studies which use vapor deposited amorphous solid water films as analogs for astrophysical icy bodies such as comets.

No abstract

The index of refraction and thickness of amorphous solid water (ASW) films are determined using laser optical interferometry. From the film thickness, the density of ASW can be calculated directly since the molecular beam flux and the H2O condensation coefficient are both known. From the index of refraction the ASW density can also be determined using the Lorentz–Lorenz relationship. The densities determined via both methods agree within experimental uncertainty. For films deposited at 22 K using a collimated molecular beam, the index of refraction and density decrease monotonically as the deposition angle is varied from normal to oblique incidence. At normal incidence the films have an index of refraction of 1.285 and are presumed to be fully dense (0.94 g/cm3). At glancing incidence (86°) the film has a refractive index of 1.05 and a density of 0.16 g/cm3, indicating a porosity exceeding 80%. The angle-dependent film density is in semiquantitative agreement with the results of ballistic deposition simulations of ASW film growth.