Potentials of physical adsorption

A molecular beam study of the adsorption and desorption of oxygen from a Pt(111) surface

Gas—surface interactions and dynamics; Thermal energy atomic and molecular beam studies

Atomic beam techniques are presently being used in many branches of surface physics such as studies of the particle-surface physisorption potential, surface structure, surface phonons, nucleation and growth on metal and insulator surfaces, surface diffusion and accommodation and sticking of molecules. This review concentrates on diffractive phenomena from surfaces, which up to now were investigated mainly with helium. The theoretical background for diffraction calculations is outlined and representative examples of different applications are given. The main subjects covered are: structural determinations of chemisorbed and physisorbed systems, investigations of disordered surfaces, selective adsorption resonances, diffusion and nucleation studies and investigations of growth and phase transitions on surfaces. Diffraction results obtained with Ne, Ar, and are also summarized.

https://iopscience.iop.org/article/10.1088/0034-4885/61/12/001/pdf

Atomic beam diffraction from solid surfaces

The dynamics of the activated dissociative chemisorption of CH4 on Ni(111) are studied by molecular beam techniques coupled with high‐resolution electron energy loss spectroscopy. The probability of the dissociative chemisorption of CH4 increases exponentially with the normal component of the incident molecule’s translational energy and with vibrational excitation. The dissociative chemisorption probability of CD4 exhibits the same trends with a large kinetic isotope effect. High‐resolution electron energy loss spectroscopy identifies the nascent products of the dissociative chemisorption event as an adsorbed methyl radical and a hydrogen atom. These results, which have shown that there is a barrier to the dissociative chemisorption, are interpreted in terms of a deformation model for the role of translational and vibrational energy in promoting dissociative chemisorption. The barrier likely arises largely from the energy required to deform the molecule sufficiently to allow a strong attractive interaction between the carbon and the Ni surface atoms. Tunneling is suggested as the final process in the C–H bond cleavage. The presence of this barrier to dissociative chemisorption presents a plausible explanation for the pressure gap in heterogeneous catalysis.

Dynamics of the activated dissociative chemisorption of CH4 and implication for the pressure gap in catalysis: A molecular beam–high resolution electron energy loss study

Observation of Direct Inelastic Scattering in the Presence of Trapping-Desorption Scattering: Xe on Pt(111)

High resolution angularly resolved time of flight distributions are presented for a supersonic argon beam scattering from a clean well‐characterized Pt(111) single crystal. A novel presentation of the resulting velocity and angular flux information in terms of iso‐flux contour maps in Cartesian velocity space allows the scattering process to be decomposed into three mutually independent directions defined by the surface normal (z), parallel to the surface and in the scattering plane (y), and parallel to the surface but perpendicular to y and z(x). The iso‐flux contour maps appear as nested ovals with principal axes oriented parallel to the above defined directions; axis length decreases in the order z, y, x. The corresponding variances in the x, y, and z velocities vary directly with the surface temperature. Three beam energy regimes are evident and are discussed in terms of the diminishing effect of the attractive well which occurs for increasing beam energies and the increasing effect of short range phenomena prevalent at high incident beam energies. Accomodation coefficients were defined and measured for the y and z directions and were 0.1 and 0.45, respectively, indicating the degree to which parallel and perpendicular momenta are not conserved in a single collision. Geometric scattering by instantaneous surface roughness is experimentally shown to be negligible for this system for beam energies <20 000 K indicating that the scattering is by the finite momentum of the surface.

Direct inelastic scattering Ar from Pt(111)

The rotationally inelastic scattering of HD(J = 0→, 1, 2, 3) from a clean Pt(111) surface is reported. Discrete peaks for each final J state are resolved in the in‐plane angular distributions, with the inelastic peaks typically being large compared to the elastic peak. Sharp modulations of these inelastic transition probabilities were found as the incident angle was varied, suggesting bound level resonances. These resonances are inconsistent with G‐vector mediated (elastic) selective adsorption, but are consistent with rotationally inelastic selective adsorption into the bound levels of the physisorption potential. Observation of bound level resonances can be used to determine molecule–surface potentials. However, unlike elastic selective adsorption, highly probable rotationally inelastic HD selective adsorption is applicable to metallic surfaces of low corrugation.

Bound level resonances in rotationally inelastic HD/Pt (111) surface scattering

Velocity distributions for Ar atoms scattering from a clean, polycrystalline tungsten surface have been measured for a wide range of incident supersonic beam energies 300 K < (1/2) m〈v2〉/k<2000 K, and surface temperatures 350 K<Ts<1900 K. This work studies directly the nature of the scattering process of an intermediate mass atom on a clean metal surface over a very wide range of conditions. Direct inelastic scattering involving a single encounter of the gas atom with the surface is the most important process. No distinct elastic or quasielastic scattering occurs. Only at the lowest temperatures is a trapping–desorption scattering process observed. The direct inelastic scattering process is characterized by the linear proportional relationship 〈KEe〉=0.83 〈KEi〉+0.20 〈KETs〉 over the entire range of energies and temperatures for 45° angle of incidence and observation in the specular direction (KEe, KEi, and KETs are the kinetic energy of the exiting Ar, the incident Ar, and the Ar in equilibrium at the surfa...

Direct measurement of velocity distributions in argon beam–tungsten surface scattering

Measurements of the velocity and angular distributions for trapping‐desorption scattering of argon from a clean, well characterized Pt(111) single crystal are reported. For certain experimental conditions, both the characteristic velocity and angular distributions deviate markedly from that predicted using equipartition arguments (i.e., a Maxwellian flux distribution in velocity and a cosine distribution in angle). The average kinetic energy for the flux exiting normal to the surface at 100 K is only 80% of that expected for a Maxwellian at TS. This kinetic energy deficit decreases and approaches zero as the detector is rotated away from the surface normal. The angular flux distribution is found to be broader than cosine. These results are discussed in terms of microscopic reversibility which permits estimates of the velocity dependent condensation coefficient to be obtained.

Jerry E. Hurst

Papers

Observation of Direct Inelastic Scattering in the Presence of Trapping-Desorption Scattering: Xe on Pt(111)

Direct inelastic scattering Ar from Pt(111)

Bound level resonances in rotationally inelastic HD/Pt (111) surface scattering

Direct measurement of velocity distributions in argon beam–tungsten surface scattering

Trapping‐desorption scattering of argon from Pt(111)