Argon Scattering off the Surface of Liquid Indium: Exit Angle and Energy Dependence

New statistical boundary conditions for argon-tungsten interactions

In this study, scattering processes of argon beam impinging on tungsten surface are investigated numerically by applying molecular dynamics (MD) simulations. Energy transfer, momentum change, and scattering processes of argon gas atoms from W(110) surface are discussed. A new model of argon-tungsten (Ar-W) interaction is proposed. Based on the new proposed model, one can simplify the boundary conditions of this problem. The new boundary conditions are proved to be in line with previous experimental and theoretical results. This paper demonstrates how to proceed normalization and further conversion of the MD simulation results into boundary conditions. Application of the new proposed boundary conditions for Ar-W interactions provides a significant speedup of computations.

A stochastic, local mode study of neon-liquid surface collision dynamics

Equations of motion for a fast, light rare gas atom passing over a liquid surface are derived and used to infer the dynamics of neon collisions with squalane and perfluorinated polyether surfaces from experimental data. The equations incorporate the local mode model of a liquid surface via a stochastic process and explicitly account for impulsive collisional energy loss to the surface. The equations predict angular distributions for scattering of neon that are in good quantitative agreement with experimental data. Our key dynamical conclusions are that experimental angular distributions derive mainly from local mode surface topography rather than from structural features of individual surface molecules, and that the available data for these systems can be accounted for almost exclusively by single collisions between neon atoms and the liquid surface.

Chemistry and Microphysics of Atmospheric Aerosol Surfaces: Laboratory Techniques and Applications

A number of current techniques are presented by which the chemistry of interaction of selected gas phase species with atmospheric surfaces as well as the microphysical behaviour of such surfaces can be investigated. The techniques discussed include (i) the coated wall flow tube reactor, (ii) the Knudsen-cell / DRIFT spectroscopy, (iii) the surface aerosol microscopy and (iv) the molecular beam scattering technique. In each of these methods specific and robust information is deduced on the kinetics and thermodynamics of gas adsorption and reaction on surfaces. Specific examples include the adsorption of acetone on ice surfaces, the adsorption and reaction of SO2 on iron oxides, the hygroscopic and phase behaviour of binary and ternary salt solution droplets (ammonium sulphate and ammonium sulphate / dicarboxylic acids solutions) as well as on the dynamics of inelastic collisions of noble gases on super-cooled sulphuric acid surfaces. In addition we also show how quantum chemistry can be utilized to assist in interpreting absorption energies on structurally different ice surfaces. Whilst each example represents different aspects of heterogenous atmospheric interactions, they jointly represent significant progress in laboratory investigations of multi-phase atmospheric chemistry with substantial potential for application to other systems and/or problems.

Collisions of noble gases with supercooled sulfuric acid-water solutions

The collisions of hyperthermal noble gases (He, Ne, Ar, Kr, Xe) with supercooled binary sulfuric acid-water mixtures (57-77 wt%) were explored in the temperature range between 210 and 240 K. The experiments were performed by directing a molecular beam of the respective gases onto a continuously renewed liquid surface and monitoring the velocity of the scattered molecules by mass spectrometry. Depending on the initial translational energies and molecular masses, we observe both inelastic scattering from the surface as well as thermalization followed by subsequent desorption. The experiments indicate that the repulsive momentum transfer in the inelastic scattering channel increases with increasing mass of the impinging gas, while it is only weakly affected by the initial velocities. The final energy of the thermally desorbing atoms can always be approximated by a Maxwell-Boltzmann distribution equal to the liquid bulk phase temperature. The influence of the binary composition of the liquid phase is only noticeable in the case of Ne, whilst this dependence diminishes for gases with molecular masses >or=40 amu. The probability of thermalisation relative to inelastic scattering increases with the bulk phase temperature, independent of the molecular masses of the colliding gas. In contrast, the fractional energy transfer during collision does not increase with temperature, except for Neon. These results can be interpreted in the model framework of hard-sphere collisions of noble gases with the surface, during which water and sulfuric acid molecules interact independently with the impinging gas.

Energy transfer at a gas-liquid interface: kinematics in a prototypical system

A detailed characterization of collisional energy transfer at a liquid surface not only provides a framework for the interpretation of experimental studies but also affords insight into energy feedback mechanisms that may be important in multiphase combustion processes. We address this problem by performing simulations of a prototypical Lennard-Jones system, investigating the dependence of the energy transfer and incident-atom trapping probability on the liquid temperature, on the mass and angle of incidence of the impinging atom, and on the strength of the gas-liquid interaction. In general, in agreement with the results of experiments, these calculations point to the dominance of kinematic effects in determining the gross energy transfer, but they also attest to the important role played by surface roughening in the enhancement of energy transfer that accompanies an increase in the liquid temperature.

Scattering of O2 from AL111

Experimental results are presented for the scattering of well-defined beams of molecular oxygen incident on clean Al(111). The data consist of scattered angular distributions measured as a function of incident angle, and for fixed incident angle, the dependence on surface temperature of the angular distributions. The measurements are interpreted in terms of a scattering theory that treats the exchange of energy between the translational and rotational motions of the molecule and the phonons of the surface using classical dynamics. The dependence of the measured angular distributions on incident beam angle and temperature is well explained by the theory. Rotational excitation and quantum excitation of the O(2) internal stretching mode are briefly discussed.

MOLECULAR BEAM STUDIES OF GAS-LIQUID INTERFACES

Molecular beam scattering experiments provide a way to disentangle the elementary steps involved in energy transfer and chemical reactions between gases and liquids. After surveying the history and recent progress in this field, we review studies of the kinematics of gas-liquid collisions and proton exchange of HCl, DCl, and HBr with supercooled sulfuric acid and liquid glycerol. These experiments help to clarify the role of the surface region in controlling trapping and interfacial- and bulk-phase reactions.