Photophysics and photochemistry of water ice films on graphite

Abstract

Our model system consists of water ice films with submonolayer to several monolayer thickness grown at ultrahigh vacuum (UHV) and low (from ∼25 K) temperatures on an atomically clean graphite (0001) surface. We characterize them experimentally with temperature programmed desorption-isothermal desorption spectroscopy (TPD-ITD) and high-resolution electron energy loss spectroscopy (HREELS) and provide examples of how ice structure and chemical properties are altered by incoming photons in broad wavelength and photon flux ranges (1.6 × 1016 - 5.5 × 1024 photons cm-2 s-1 for λ = 220-700 nm). The phenomena of "photon annealing" of pure amorphous films and their structure-selective laser ablation are discussed. In the case of water ice doped with alkali metals and coadsorbed with NO, we report on the observations of photoreactions taking place on the graphite-ice interface. Depending on the composition, irradiation leads to formation of H2, CH4, CO, NH3, and CO2. In all examples the attention is on the operating mechanism for the transformations. The role of photo-excited charge carriers in the graphite substrate is underlined. Copyright 2004 by the American Geophysical Union.