Frank E. Livingston

Surface and bulk diffusion of HDO on ultrathin single-crystal ice multilayers on Ru(001)

The kinetics of HDO surface and bulk diffusion on ultrathin (25–192 BL; 90–700 A) single-crystal H216O ice multilayers were studied using a combination of laser-induced thermal desorption (LITD) probing and isothermal desorption depth-profiling. The single-crystal hexagonal ice multilayers were grown epitaxially on a single-crystal Ru(001) metal substrate with the basal (001) facet of ice parallel to the Ru(001) surface. HDO surface diffusion on the single-crystal ice multilayer was not observed within the resolution of the LITD experiment at T=140 K. These LITD surface diffusion experiments yielded an upper limit to the HDO surface diffusion coefficient of Ds⩽1×10−9 cm2/s at T=140 K. The bulk diffusion coefficients were measured along the c axis of the hexagonal ice crystal which is perpendicular to the (001) plane. HDO was observed to diffuse readily into the underlying H216O ice multilayer. The measured HDO bulk diffusion coefficients ranged from D=2.2(±0.3)×10−16 cm2/s to D=3.9(±0.4)×10−14 cm2/s over ...

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 A) 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 ≤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.

DYNAMIC ICE SURFACE IN THE POLAR STRATOSPHERE

Heterogeneous reactions on polar stratospheric clouds (PSCs), such as ClONO2+HCl — Cl2+HNO3, are important for an understanding of the production of active chlorine species (HOCl, Cl2) and Antarctic ozone depletion. H2O-ice forms the Type II PSCs and the nature of H2O-ice surfaces under stratospheric conditions may affect the heterogeneous chemistry. This review focuses on recent measurements of H2O adsorption kinetics on ice, H2O desorption kinetics from ice, H2O surface diffusion on ice and H2O diffusion into ice. These measurements reveal that the ice surface is extremely dynamic under polar stratospheric conditions. For example, the residence time for an H2O molecule on an ice surface at 188 K is only ~20 milliseconds before desorption and only ~0.4 milliseconds before diffusion into the ice bulk. The dynamic nature of the ice surface may significantly affect the adsorption, solvation, diffusion and reaction of the chlorine reservoir molecules (ClONO2, HCl). The dynamic ice surface may also serve as a model for the surfaces of other molecular solids.

Optimization of a rotary Q-switched Er:YAG laser

An erbium:yttrium–aluminum–garnet Er:YAG (λ=2.94 μm) rotary Q-switched laser was optimized for long-term stability and reliability, maximum output energy, and TEMoo mode quality. This optimization was achieved employing a close-coupled BaSO4 diffuse reflector pump chamber and a dehumidifying assembly and an ultra-dry-air purge system. The performance and efficiency of the Er:YAG laser were further enhanced by appropriate variations in the coolant temperature, rotational frequency of the Q-switch mirror, and pulse repetition rate. These improvements should facilitate the implementation of rotating mirror Q-switched Er:YAG lasers in various laser photoablation and depth-profiling applications.