Electron-stimulated reactions and O2 production in methanol-covered amorphous solid water films

Abstract

The low-energy, electron-stimulated desorption (ESD) of molecular products from amorphous solid water (ASW) films capped with methanol is investigated versus methanol coverage (0-4x10(15) cm(-2)) at 50 K using 100 eV incident electrons. The major ESD products from a monolayer (ML) of methanol on ASW are quite similar to the ESD products from bulk methanol film: H(2), CH(4), H(2)O, C(2)H(6), CO, CH(2)O, and CH(3)OH. For 40 ML ASW films, the molecular oxygen, hydrogen, and water ESD yields from the ASW are suppressed with increasing methanol coverage, while the CH(3)OH ESD yield increases proportionally to the methanol coverage. The suppression of the water ESD products by methanol is consistent with the nonthermal reactions occurring preferentially at or near the ASW/vacuum interface and not in the interior of the film. The water and molecular hydrogen ESD yields from the water layer decrease exponentially with the methanol cap coverage with 1/e constants of approximately 6 x 10(14) and 1.6 x 10(15) cm(-2), respectively. In contrast, the O(2) ESD from the water layer is very efficiently quenched by small amounts of methanol (1/e approximately 6.5 x 10(13) cm(-2)). The rapid suppression of O(2) production by small amounts of methanol is due to reactions between CH(3)OH and the precursors for the O(2)-mainly OH radicals. A kinetic model for the O(2) ESD, which semiquantitatively accounts for the observations, is presented.