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.

Scattering of CO and N2 molecules by a graphite surface.

Measurements of angular distributions for the scattering of well-defined incident beams of CO and N(2) molecules from a graphite surface are presented. The measurements were carried out over a range of graphite surface temperatures from 150 to 400 K and a range of incident translational energies from 275 to over 600 meV. The behavior of the widths, positions and relative intensities of the angular distributions for both CO and N(2) were found to be quite similar. The experimental measurements are discussed in comparison with calculations using a classical mechanical model that describes single collisions with a surface. Based on the behavior of the angular distributions as functions of temperature and incident translational energy, and the agreement between measured data and calculations of the single-collision model, it is concluded that the scattering process is predominantly a single collision with a collective surface for which the effective mass is significantly larger than that of a single carbon atom. This conclusion is consistent with that of earlier experiments for molecular beams of O(2) molecules and Xe atoms scattering from graphite. Further calculations are carried out with the theoretical molecular scattering model in order to predict translational and rotational energy transfers to and from the molecule during scattering events under similar initial conditions.

Scattering of O2 from a graphite surface.

Recently an extensive series of measurements has been presented for the angular distributions of oxygen molecules scattered from a graphite surface. Incident translational energies ranged from 291 to 614 meV with surface temperatures from 150 to 500 K. The measurements were taken with a fixed angle of 90° between the source beam and the detector and the angular distributions consisted of a single broad peak with the most probable intensity located at an angle slightly larger than the 45° specular position. Analysis with the hard cubes model for atom-surface scattering indicated that the scattering is primarily a single collision event with a surface having a collective effective mass much larger than a single carbon atom. Limited analysis with a classical diatomic molecular scattering theory was also presented. In this paper a more complete analysis using the classical diatomic molecular scattering theory is presented. The energy and temperature dependence of the observed angular distributions are well described as single collision events with a surface having an effective mass of 1.8 carbon graphite rings. In agreement with the earlier analysis and with other experiments, this suggests a large cooperative response of the carbon atoms in the outermost graphene layer.

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.

Scattering of Xe from graphite.

Recently a series of experimental measurements for the scattering of Xe atoms from graphite has been reported for both energy-resolved spectra and angular distributions. This system is of fundamental interest because the projectile Xe atoms are considerably more massive than the carbon atoms making up the graphite surface. These measurements were initially analyzed using the hard cubes model and molecular dynamics simulations, and both treatments indicated that the scattering process was a single collision in which the incoming Xe atom interacted strongly with a large number of carbon atoms in the outermost graphite layer. In this work we analyze the data using a single scattering theory that has been shown to explain a number of other experiments on molecular beam scattering from surfaces. These calculations confirm that the scattering process is a single collision with an effective surface mass that is substantially larger than that of the basic graphite ring.

Angular intensity distribution of a molecular oxygen beam scattered from a graphite surface.

The scattering of the oxygen molecule from a graphite surface has been studied using a molecular beam scattering technique. The angular intensity distributions of scattered oxygen molecules were measured at incident energies from 291 to 614 meV with surface temperatures from 150 to 500 K. Every observed distribution has a single peak at a larger final angle than the specular angle of 45° which indicates that the normal component of the translation energy of the oxygen molecule is lost by the collision with the graphite surface. The amount of the energy loss by the collision has been roughly estimated as about 30-41% based on the assumption of the tangential momentum conservation during the collision. The distributions have also been analyzed with two theoretical models, the hard cubes model and the smooth surface model. These results indicate that the scattering is dominated by a single collision event of the particle with a flat surface having a large effective mass. The derived effective mass of the graphite surface for the incoming oxygen is 9-12 times heavier than that of a single carbon atom, suggesting a large cooperative motion of the carbon atoms in the topmost graphene layer.

He/Ar-atom scattering from molecular monolayers: C60/Pt(111) and graphene/Pt(111).

Supersonic He and Ar atomic beam scattering from C(60) and graphene monolayers adsorbed on a Pt(111) surface are demonstrated in order to obtain detailed insight into a gas-molecule collision that has not been studied in detail so far. The effective masses and phonon spectral densities of the monolayers seen by different projectiles are discussed based on classical models such as the hard cube model and the recently developed smooth surface model. Large effective masses are deduced for both the monolayers, suggesting collective effects of surface atoms in the single collision event. The effective Debye temperature of graphene was found to be similar to that reported in highly oriented pyrolytic graphite (HOPG), indicating that the graphene is decoupled well from the Pt substrate. A much smaller Debye-Waller factor was found for the C(60) layer, probably reflecting the strong C(60)-Pt(111) interaction.

Rare gas collisions with molten metal surfaces.

Newly available experimental data for the scattering of argon, neon, and xenon atoms from molten gallium, indium, and bismuth surfaces are compared to calculations with classical scattering theory. The results of the theory are in reasonable agreement with observed energy-resolved spectra taken at fixed angles, with in-plane angular distributions, and with the first available out-of-plane angular distribution spectra for these systems. For all three of the rare gases, scattering from liquid Ga required the use of an effective surface mass equal to 1.65 times the mass of a single Ga atom. The need for a larger effective mass has been noted previously for Ar/Ga scattering and is indicative of collective effects in the liquid Ga. Comparisons with data taken at low incident energies enable estimates of the physisorption well depth in the interaction potentials for many of the gas-metal combinations.

The diagnosis of onychomycosis in a geriatric population: a study of 450 cases in South Florida.

An investigative study was performed to determine the diagnosis of onychomycosis in a South Florida geriatric population. In this study, 450 cases of suspected onychomycosis involving men and women 65 years of age and older from a private practice office and two nursing home settings were used. Samples were taken from the hallux toenail and sent to a mycology laboratory for fluorescent potassium hydroxide (KOH) preparation and microscopic examination of a fungal culture. Of the 450 cases studied, 46.4% of the patients had a single fungal organism cultured, 30.4% had a mixed fungal infection cultured, and 23.1% had no fungal growth. Saprophytes were found in 59.9% of the 526 total fungal organisms cultured while dermatophytes were found in only 23.8%. The results of this investigation demonstrate that there may be a shift from isolated dermatophyte infection to mixed saprophyte infections in a geriatric population with onychomycosis.

Effects of Microfluidizer technology on Bacillus licheniformis spores in ice cream mix.

We studied the effect of Microfluidizer technology (sometimes referred to as "microfluidization"), a new ultra-high pressure homogenization process, on spores of Bacillus licheniformis in ice cream mix. Four batches of pasteurized ice cream mix were preheated to 33, 36, 44, or 50 degrees C, and spores of B. licheniformis were added to yield an inoculum of 2.0 x 10(4) spores/ ml of mix. Samples were treated at 50,000, 100,000, 150,000, and 200,000 kPa. Respective percentages of spore destruction ranged from 6 to 68%. As process pressure in the Microfluidizer system increased, the temperature of the product also increased. At the Microfluidizer system outlet, temperatures ranged from 46 to 88 degrees C. Therefore, a combination of forces, including high pressure and temperature, likely had a multiplier effect on spore destruction during Microfluidizer processing of ice cream mix. Data suggest that it might be possible to design a pasteurizer-Microfluidizer system that would inactivate most bacterial spores in dairy foods without the extreme heat treatment currently required in commercial processing operations.