Origin of non-zero-order H 2O desorption kinetics from crystalline ice multilayers on Ru(001)

Abstract

Recent studies have reported non-zero-order kinetics for H 2O desorption from crystalline ice multilayers on Ru(001). To understand the origin of the observed non-zero-order kinetics, D 2O desorption from ultrathin (15–135 BL; 55–490 Å) crystalline D 2O ice multilayers on Ru(001) was measured using a novel combination of laser-induced thermal desorption (LITD) spatial probing and isothermal desorption flux analysis. The ice multilayers were grown on a single-crystal Ru(001) metal substrate using either backfill D 2O vapor deposition or multichannel capillary array dosing methods. The ice multilayers grown via backfill vapor deposition were smooth and highly uniform. The LITD and isothermal desorption flux studies demonstrated that D 2O desorption from these uniform ice multilayers exactly followed zero-order kinetics. Slight deviations from zero-order desorption kinetics were observed only at low D 2O coverages of ≤5 BL D 2O and were attributed to enhanced D 2O-Ru(001) substrate interactions or slight ice surface roughening. In contrast, the ice films prepared using capillary array dosing were spatially non-uniform and exhibited a decreasing multilayer coverage versus distance from the center of the substrate. This initial non-uniform D 2O coverage distribution had a dramatic impact on the isothermal desorption flux measurements and produced non-zero-order desorption kinetics. The deviations from zero-order kinetics were directly related to changes in the ice film surface area as the non-uniform initial multilayer coverage was completely desorbed at various positions on the Ru(001) substrate at different times. The desorption kinetics of D 2O from smooth and uniform D 2O ice multilayers on Ru(001) are strictly zero-order. The previous reports of non-zero-order kinetics are assigned to a non-uniform initial D 2O multilayer coverage distribution.