PNNL

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 - 4 x 1015 cm-2) at 50 K using 100 eV incident electrons. The major ESD products from a monolayer of methanol on ASW are quite similar to the ESD products from bulk methanol film: H2, CH4, H2O, C2H6, CO, CH2O, and CH3OH. For 40 ML ASW films, the molecular oxygen, hydrogen, and water ESD yields from the ASW are suppressed with increasing methanol coverage, while the CH3OH ESD yield increases proportionally to the methanol coverage. The suppression of the water ESD products by methanol is consistent with the non-thermal 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 ~ 0.6 x 1015 cm-2 and 1.6 x 1015 cm-2, respectively. In contrast, the O2 ESD from the water layer is very efficiently quenched by small amounts of methanol (1/e ~ 6.5 x 1013 cm-2). The rapid suppression of O2 production by small amounts of methanol is due to reactions between CH3OH and the precursors for the O2 - mainly OH radicals. A kinetic model for the O2 ESD which semi-quantitatively accounts for the observations is presented.