Howard Saltsburg

Measurements of the Reflection, Adsorption, and Desorption of Gases from Smooth Metal Surfaces

The scattering of thermal energy beams of 4He, H2, and D2 from the (111) surfaces of epitaxially grown Ni and Ag single crystals has been measured. Also measured were the angular dependences of the desorption of HD resulting from the dissociation–recombination exchange reaction of H2 and D2 on active (111) Ni surfaces. The angular desorption dependence of HD is contrary to the Knudsen law and is describable analytically by cosdθr where 2.5 < d < 4.0 depending on the condition of the particular surface under study. Comparison of these results with the complementary nonreacted scattering distributions, i.e., direct reflection on (111) Ni, indicates that the two measured components, reflection and desorption, combine to yield the total scattered flux from the surface. An artificial synthesis of the total flux distribution expected from the surface assuming an isotropic hydrogen source and the measured reflection and desorption distributions yields a cosine dependence, as expected, at equilibrium. The reflect...

Comparisons of the Reflection and Diffraction of 3He, 4He, H2, and D2 from the (001) Surface of LiF

Comparisons of the reflection and diffraction of thermal energy beams of 3He, 4He, H2, and D2 from the (001) surface of LiF are reported. Intense diffracted beams, including those of the second order, result from a freshly cleaved LiF surface without heating, and large exposures of H2O vapor do not reveal any measurable change in either the intensity, normalized to constant incident beam intensity, or location of such beams. The intensity and location of the individual beams are examined, taking into account the influence of adjacent beams. The locations of beams not so influenced display good agreement with the predictions of simple diffraction theory. The shapes and intensities of the diffracted beams are compared for the four gases with reference to possible inelastic effects. The decreasing intensities of diffracted beams for the same incident de Broglie wavelength are found to be in the order 4He, H2, and D2, i.e., in the order of increasing rotational energy coupling with the surface; 3He diffracted...

Preferential Scattering of Ar from LiF: Correlation with Lattice Properties

A study of the scattering of thermal energy Ar atoms from the (001) plane of LiF is reported. Multiple peaks in the scattered flux distribution are observed. The major peak in the distribution is the analog of the single lobe directed scattering distributions observed previously, using metal targets, in terms of its behavior as a function of angle of incidence and beam temperature. However, this major peak in the Ar/LiF data also displays a dependence on the azimuthal angle of the target which may be associated with the anisotropy in the elastic properties of LiF. Other peaks in the Ar distribution are observed to lie in fixed directions along rows of second‐nearest‐neighbor unlike ions in the LiF lattice. It is suggested that dipole–induced‐dipole attractive forces serve to enhance the coupling of the Ar atoms to the longitudinal optical modes of vibration in these directions, giving rise to preferential re‐emission.

Scattering of 3He, 4He, and the Hydrogenic Molecules from the (111) Plane of Ag

Molecular beams of the isotopes of He and H have been reflected from the (111) plane of epitaxially grown Ag crystals. The spatial scattering distributions of 3He and 4He are compared with those of H2, HD, and D2 at various angles of incidence and primary beam temperatures from 80° to 1500°K. Deviations in the specularity of these gases are interpreted in terms of the exchange of rotational state energy of the diatomic species at the surface. Comparisons of number density and flux measurements indicate a significant dependence of the mean speed of the reflected beam on θr. It is proposed that this variation is also enhanced by the degree of rotational state interactions, which decreases with increasing beam temperature. Finally, it is suggested that the appearance of a truly specular component in these scattering distributions is related to an apparent surface roughness through the relation λ > h cosθi, where 0.8 A < h < 1.7 A.

Intensity data for the reflection and diffraction of 3He, 4He, H2 AND D2 From the (001) Surface of LiF

Abstract Intensity data for the reflection and diffraction of incident thermal beams of 3 He, 4 He, H 2 , and D 2 from the (001) surface of LiF are presented and compared with a recent theory of diffraction. Particular emphasis is placed on the variation of the intensity of the specular beam with angle of incidence, incident beam temperature, and azimuthal orientation. Also reported are intensity data for the resolved diffracted beams both in and out of the plane of incidence. Qualitative agreement is found to exist between theory and experiment for many of the intensity variations studied. Lack of total agreement is probably due to the inadequacy of the theory in not considering inelastic processes and the failure to account for the particles in the beams which would theoretically be diffracted into the surface but which physically must leave the surface as part of the scattered back- ground. Also to be considered is the large discrepancy between values of the bound energy states for 3 He used in the theory and those measured experimentally. Any comparison of H 2 or D 2 scattering with the theory is seriously in doubt because of the contribution to inelastic scattering due to rotational energy state coupling of the diatomic molecules with the surface.

Scattering of Velocity‐Filtered Atomic Beams of Ar and Xe from the (111) Plane of Silver

The scattering of nearly monoenergetic atomic beams of Ar and Xe from the (111) plane of silver has been studied as a function of the nominal velocity υ0 transmitted by a slotted‐disk velocity selector (SDVS) used as a velocity filter on the incident thermal‐energy (Maxwellian) beam. The selector has a velocity spread of ±0.19 υ0 and studies were carried out over a range of υ0 from 2.2 × 104 to 5.3 × 104 cm/sec. The scattered beam distributions were found to be directed, corresponding closely to those of Maxwellian beams when υ0 = ῡ = 34(2πkTB / M)1 / 2, the average velocity of the corresponding Maxwellian beam of temperature TB. These results, together with the results of earlier scattering studies, imply that the thermal motion of the lattice is the dominant factor in producing the spatial dispersion as well as the velocity dispersion in the scattered beam that has been observed by other investigators. The most likely origin of these dispersive effects is the languidness of the collision in the sense us...

Asymmetry of Inner Electron Distributions in Homonuclear Diatomic Molecules

Disturbances in the distributions of inner shell electrons due to bond formation in homonuclear diatomic molecules have been investigated using a Fermi‐Thomas type statistics. In the majority of molecules examined the disturbance seems to penetrate only to the shell directly beneath the valence shell.Plots of the number of electrons disturbed versus atomic number are periodic, the disturbance being less for the alkali molecules than for the halogens. In any one group, these plots are remarkably linear. This behavior has permitted us to calculate the bond distance in Rb2. It is found to be 4.28A. This value is to be compared with the distance 4.32A between nearest neighbors in the metal.