Examination of liquid metal surfaces through angular and energy measurements of inert gas collisions with liquid Ga, In, and Bi

Determination of the surface corrugation amplitude from classical atom scattering

The energy landscape of an atomic or molecular projectile interacting with a surface is often described in terms of a corrugation function that gives the classical turning point as a function of position vector parallel to the surface. It is shown here that the relative height variation of the corrugation function for scattering of atoms under classical conditions can be determined by a measurement of the maximum intensity in energy-resolved scattering spectra as a function of surface temperature. This is demonstrated by developing a semiclassical quantum theory of atomic scattering from corrugated surfaces and then extending the theory to the classical limit of large incident energies and high surface temperatures. Comparisons of calculations with available data for Ar atom scattering determine the corrugation amplitude for a molten In surface to be about 29% of the mean interparticle spacing in the bulk liquid.

Relating temperature dependence of atom scattering spectra to surface corrugation

It is suggested that a measurement of the temperature dependence of the most probable intensity of energy-resolved atom-surface scattering spectra can reveal the strength of the surface corrugation. To support this conjecture, a classical mechanical theory of atom scattering from a corrugated surface, valid in the weak corrugation limit, is developed. The general result for the scattering probability is expressed in terms of spatial integrals over the impact parameter within a surface unit cell. For the case of a one-dimensional corrugation, approximate expressions for the scattering probability are obtained in terms of analytic closed form expressions. As an indicator of its relation to experimental measurements, calculations using a one-dimensional corrugation model are compared with data for Ar scattering from a molten Ga surface and an approximate value of the corrugation height parameter is extracted.

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.

Calculations of trapping and desorption in heavy atom collisions with surfaces

Calculations are carried out for the scattering of heavy rare gas atoms with surfaces using a recently developed classical theory that can track particles trapped in the physisorption potential well and follow them until ultimate desorption. Comparisons are made with recent experimental data for xenon scattering from molten gallium and indium, systems for which the rare gas is heavier than the surface atoms. The good agreement with the data obtained for both time-of-flight energy-resolved spectra and for total scattered angular distributions yields an estimate of the physisorption well depths for the two systems.

Quantum-State-Resolved CO2 Scattering Dynamics at the Gas−Liquid Interface: Dependence on Incident Angle

Energy transfer dynamics at the gas-liquid interface have been probed with a supersonic molecular beam of CO2 and a clean perfluorinated-liquid surface in vacuum. High-resolution infrared spectroscopy measures both the rovibrational state populations and the translational distributions for the scattered CO2 flux. The present study investigates collision dynamics as a function of incident angle (thetainc = 0 degrees, 30 degrees, 45 degrees, and 60 degrees), where column-integrated quantum state populations are detected along the specular-scattering direction (i.e., thetascat approximately thetainc). Internal state rovibrational and Doppler translational distributions in the scattered CO2 yield clear evidence for nonstatistical behavior, providing quantum-state-resolved support for microscopic branching of the gas-liquid collision dynamics into multiple channels. Specifically, the data are remarkably well described by a two-temperature model, which can be associated with both a trapping desorption (TD) component emerging at the surface temperature (Trot approximately TS) and an impulsive scattering (IS) component appearing at hyperthermal energies (Trot > TS). The branching ratio between the TD and IS channels is found to depend strongly on thetainc, with the IS component growing dramatically with increasingly steeper angle of incidence.

Collisions and reactions of gaseous propanol with molten NaOH∕KOH

Molecular beam scattering experiments are used to investigate collisions of a protic molecule, deuterated 1-propanol (PrOD), with an extremely basic solvent, the 5149 mol % NaOH/KOH eutectic mixture. This powerful deprotonating medium readily absorbs PrOD from the gas phase. Nearly all PrOD molecules that thermalize at the surface of the melt enter the liquid and dissolve for long times, most likely residing as PrO- after deprotonation by OH-. The PrO- solvation time is controlled by dissolved H2O, which reprotonates the anion and liberates D --> H exchanged PrOH. We find no evidence for decomposition of the alcohol; at the 463 K temperature of the experiments, the hydroxide solution appears to store propanol reversibly.

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.

Quantum State-Resolved Energy Transfer Dynamics at Gas−Liquid Interfaces: IR Laser Studies of CO2 Scattering from Perfluorinated Liquids

An apparatus for detailed study of quantum state-resolved inelastic energy transfer dynamics at the gas-liquid interface is described. The approach relies on supersonic jet-cooled molecular beams impinging on a continuously renewable liquid surface in a vacuum and exploits sub-Doppler high-resolution laser absorption methods to probe rotational, vibrational, and translational distributions in the scattered flux. First results are presented for skimmed beams of jet-cooled CO(2) (T(beam) approximately 15 K) colliding at normal incidence with a liquid perfluoropolyether (PFPE) surface at E(inc) = 10.6(8) kcal/mol. The experiment uses a tunable Pb-salt diode laser for direct absorption on the CO(2) nu(3) asymmetric stretch. Measured rotational distributions in both 00(0)0 and 01(1)0 vibrational manifolds indicate CO(2) inelastically scatters from the liquid surface into a clearly non-Boltzmann distribution, revealing nonequilibrium dynamics with average rotational energies in excess of the liquid (T(s) = 300 K). Furthermore, high-resolution analysis of the absorption profiles reveals that Doppler widths correspond to temperatures significantly warmer than T(s) and increase systematically with the J rotational state. These rotational and translational distributions are consistent with two distinct gas-liquid collision pathways: (i) a T approximately 300 K component due to trapping-desorption (TD) and (ii) a much hotter distribution (T approximately 750 K) due to "prompt" impulsive scattering (IS) from the gas-liquid interface. By way of contrast, vibrational populations in the CO(2) bending mode are inefficiently excited by scattering from the liquid, presumably reflecting much slower T-V collisional energy transfer rates.