Ammonia Adsorption on Crystalline Ice Surface: Adsorption Structure and Site Selectivity

Abstract

The structural heterogeneity of an ice surface at the atomic level is an important characteristic that influences the interactions between adsorbate molecules and the ice surface. In this study, we investigated the adsorption of ammonia on the basal (0001) surface of crystalline ice (CI) films to examine the adsorption selectivity of ammonia molecules on atomistically different sites of the ice surface. The adsorption structure and site-selectivity of ammonia adsorption on ice surfaces were examined by conducting temperature-programmed desorption, low energy sputtering, reactive ion scattering, reflection- absorption infrared spectroscopy, and surface voltage measurements. Ammonia adsorbs on ice surfaces without undergoing protonation, dissociation, or penetration into the bulk ice. At low coverages, adsorption occurs exclusively at the dangling H atom sites of the surface via N center dot center dot center dot H-O bond formation. The average orientation of the adsorbed molecular dipoles leans substantially toward the surface. After saturation of the dangling H sites, further adsorption of ammonia leads to the formation of a multilayer with isotropic molecular orientation. Titration experiment for the surface population of dangling H sites with ammonia reveals that the surface density of free O-H groups is 0.29 +/- 0.04 monolayers for the CI films prepared by water vapor deposition at 140 K and postannealing at 160 K, indicating that the basal ice surface is not significantly reconstructed under vacuum conditions.