Origin of non-zero-order H2O desorption kinetics from crystalline ice multilayers on Ru(001)

Recent studies have reported non-zero-order kinetics for H2O desorption from crystalline ice multilayers on Ru(001). To understand the origin of the observed non-zero-order kinetics, D2O desorption from ultrathin (15-135 BL; 55-490 Angstrom) crystalline D2O 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 D2O 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 D2O desorption from these uniform ice multilayers exactly followed zero-order kinetics. Slight deviations from zero-order desorption kinetics were observed only at low D2O coverages of less than or equal to 5 BL D2O and were attributed to enhanced D2O-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 D2O 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 D2O from smooth and uniform D2O ice multilayers on Ru(001) are strictly zero-order. The previous reports of non-zero-order kinetics are assigned to a non-uniform initial D2O multilayer coverage distribution. (C) 1999 Elsevier Science B.V. All rights reserved.