Joe N. Smith

Molecular Beam Study of CO Oxidation on a (111) Platinum Surface.

The heterogeneous oxidation of CO on epitaxially grown Pt(111) surfaces has been investigated using modulated molecular beam techniques. Within the range of pressures from 10−7 to 10−5 torr and temperatures from 450 to 950°K, the overall reaction is apparently first order. The observance of higher order kinetics in these experiments is argued to be an artifact of the beam technique used here and suggests a high surface diffusion rate for adsorbed CO. The measured angular dependence of the CO2 desorption rate from the Pt(111) surfaces does not follow the Knudsen law of random (cosine) desorption, but is much more sharply peaked about the surface normal. As with the desorption of hydrogen, the desorption law for CO2 is given by cosdθr where in this case d ≤ 6, depending on the condition of the surface. The adsorption of CO is also shown to have an angular dependence and is not activated on Pt(111). The results of this study strongly suggest that a Langmuir‐Hinshelwood mechanism dominates the reaction, with ...

Reflection and Dissociation of H2 on Tungsten

Through the use of modulated atomic beam techniques, the reflection and dissociation of H2 at a tungsten surface above 2500°K has been studied. Two experimental configurations were employed. In the first configuration, a collimated beam of molecular hydrogen was directed at the tungsten surface, and the angular distribution of particles evaporating from the surface was examined mass spectrometrically. In the second configuration, the tungsten test surface was located in the first of three differentially pumped vacuum chambers. H2 was admitted into this chamber, thereby providing an isotropic source of incident particles. Particles evaporating from the surface were collimated and examined mass spectrometrically in the third chamber. From these experiments it is observed that at high temperatures the angular distribution of atomic hydrogen evaporating from the target surface obeys the cosine law. However, the angular distribution of reflected H2 displays a shift toward the specular ray. Above 2500°K the sti...

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...

Molecular Beam Study of the Oxidation of Deuterium on a (111) Platinum Surface

The reaction D2+12O2→D2O occurring on a (111) Pt surface has been investigated using modulated molecular beam techniques. With a molecular beam of D2 and a (111) Pt surface immersed in an O2 ambient, the production of D2O was studied as a function of D2 flux and temperature, O2 pressure, Pt temperature and the angle of incidence of the D2 beam. From these data the rate of production of D2O can be expressed by a reaction rate proportional to PD 2 2.0 PO 2 0.8  exp[−3.6 (kcal/mole) / kTD 2]  exp[−12 (kcal/mole) / kTPt] in the pressure and temperature regime PD 2 ∼ 10 −7 torr, PO 2 ∼ 10 −5 torr, TD 2 ∼ 1200°K, and TPt ∼ 700°K. Further, a reaction mechanism which is consistent with this rate expression and also with the observed angular dependence is presented.

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...

Molecular‐Beam Scattering from the (111) Plane of Silver

The spatial distributions of molecular beams of CH4, NH3, He, Ne, Ar, and Xe scattered from the (111) plane of an epitaxially grown Ag single crystal have been examined both during growth of the Ag crystal and immediately afterward, and no distinction between these modes was observable. The scattering of Ne, CH4, and NH3 has been compared to illustrate the important role of the gas—metal interaction energy; NH3, the most strongly interacting gas, gives rise to nearly diffuse scattering, while Ne is nearly specularly directed. Although the scattering of He is essentially specularly directed at all incident angles and temperatures, the heavier gases (Ar and Xe) exhibit scattering maxima lying between the specular direction and the tangent to the surface (supraspecular scattering), with the location of the maxima depending in a complex manner on the angle of incidence of the beam and the beam energy. Since a necessary condition for supraspecularity is that the beam temperature be greater than the surface tem...

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.

RARE-GAS SCATTERING FROM LiF: CORRELATION WITH LATTICE PROPERTIES. II.

The scattering of thermal energy atomic beams of Ne, Ar, Kr, and Xe from the (001) face of LiF has been studied. Two dominant peaks in the scattering distributions in the plane of incidence are observed: one peak remains fixed with respect to the crystal whereas the position of the other displays a strong dependence on angle of incidence, incident energy, and azimuthal orientation of the target. The former peak is associated with the normal modes of the lattice, in particular with dipole–induced dipole attractive forces produced by longitudinal optical modes in next‐nearest‐neighbor unlike ion directions. The latter peak is discussed in terms of the surface properties of the crystal, e.g., interaction potential and elastic properties, the latter acting on the transfer of tangential momentum. For Ne, additional peaks occur which arise from diffraction effects. The present results provide additional insight into the phenomenon of preferential scattering reported earlier and strongly emphasize the inadequaci...

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.

Molecular beam scattering from solid surfaces: A critical review

Abstract This review covers experimental studies of gas-surface scattering reported in the period from approximately 1969 through the first half of 1972. It emphasizes rare gas scattering from a variety of metal single crystals at incident energies from about 0.025 eV to nearly 20 eV, and includes a discussion of the various scattering regimes in that energy range. In addition, light gas scattering from LiF and certain highly structured metal surfaces is surveyed. The observation of single-phonon inelastic events are described. Recent studies of desorption phenomena are reviewed. A correlation of the various results is made and areas in which questions remain are indicated. Apart from parametric correlations, no attempt is made to assess the validity of the various theoretical models that have been developed to describe the scattering process.