The effect of the incident collision energy on the porosity of vapor-deposited amorphous solid water films.

Molecular beam techniques are used to grow water films on Pt(111) with various incident angles and collision energies from 5 to 205 kJ/mol. The effect of the incident angle and collision energy on the porosity and surface area of the vapor-deposited water films was studied using nitrogen physisorption and infrared spectroscopy. At low incident energy (5 kJ/mol), the infrared spectra, which provide a direct measure of the surface area, show that the surface area increases with incident angle and levels off at angles > 65 degrees . This is in contrast to the nitrogen uptake data, which display a maximum near 65 degrees because of the decrease in nitrogen condensation in the larger pores that develop at high incident angles. Both techniques show that the morphology of vapor-deposited water films depends strongly on the incident kinetic energy. These observations are consistent with a ballistic deposition shadowing model used to describe the growth of highly porous materials at glancing angle. The dependence of film morphology on incident energy may have important implications for the growth of porous materials via glancing angle deposition and for the structure of interstellar ices.