T.N. Rhodin

Synchrotron radiation study of chemisorptive bonding of CO on transition metals — Polarization effect on Ir(100)☆

Abstract Photoemission spectra in the photon energy range 15 eV to 30 eV on clean Ir(100) show considerable dependence of spectral structure on photon energy. In addition, phtoionization cross-sections for the 4σ, 1π and 5σ levels of chemisorbed CO are strongly dependent on the polarization of the incident radiation. Finally, analysis of a broad body of published data on CO chemisorption for many metals in addition to that of this study leads to the conclusion that the separation in energy between the 4σ and 1π peaks of chemisorbed CO varies systematically with the position of the adsorbent in the Periodic Table. This is ascribed to a systematic geometrical stretching of the bonds in the adsorbate. The susceptibility of the CO molecule to discociation upon chemisorption is also found to vary systematically depending on the position of the adsorbent in the Periodic Table.

Chemisorption on (001), (110) and (111) nickel surfaces: A correlated study using LEED spectra, Auger spectra and work function change measurements☆

Abstract An experimental study of the chemisorption of a variety of closely related absorbates (H, O, C, CO, S, Se and Te) on clean single crystal nickel surfaces is presented. The nature of chemisorption is investigated through a correlated study of the modifications in low energy electron diffraction (LEED) patterns and intensity-energy spectra, Auger electron energy distributions, and work function change measurements which occur as a function of coverage and heat treatment. These experimental features are used to categorize the observed chemisorption phenomena into three distinct groups: (1) Molecular-like adsorption associated with relatively weak bonding, characteristic of carbon monoxide on (001), (110) and (111) nickel and oxygen and sulfur on (111) nickel. The adsorption is associated with attenuation of the LEED spectra intensities where only relatively minor changes occur in the structure and position of peaks in the integral order beams from that of the corresponding clean surfaces. The occurrence of relatively larger work function changes (∼ 1 eV) is characteristic of this group. (2) Atomic-like adsorption associated with strong bonding, but negligible substrate disruption, characteristic of oxygen, sulfur, selenium, tellurium and carbon on (001) Ni and for relatively low coverages (≲ 0.5 monolayers) of oxygen on (110) Ni. Here strong modifications of the specular beam LEED intensity-energy features occur with only slight modification of the other integral order beams from those of the clean surface. The surface dipole is smaller than in case (1) and produces a relatively small work function change (≲ 0.5 eV). (3) Disruptive adsorption associated with possible distortion and/or penetration of the lattice. This is characteristic of adsorption temperatures above room temperature and/or higher coverages (≳ 0.5 monolayers) of oxygen on both (001) and (110) surfaces, for hydrogen exposure to (001) and to (110) nickel, and to a lesser extent, possibly sulfur on (001) and (110) nickel. The LEED intensity-energy spectra show strong modifications in the integral order beam lineshapes from that of the clean surface. Associated work function changes again are small (∼ 0.50 eV). In summary, chemisorption of reactive gases on nickel is dependent on specific factors including the adsorbate, the surface crystallography, the adsorption temperature and the intensity of exposure. Specific variations among these factors correlate with large variations in the adsorption process from superficial bonding to the surface at one extreme to significant surface penetration and lattice disruption at the other.

Binding and charge transfer associated with alkali metal adsorption on single crystal nickel surfaces

Abstract Work function and thermal desorption energy measurements of alkali metal coated single crystal Ni surfaces are reported and correlated with previously reported surface crystallography measurements. It is shown that the minimums in the work function versus coverage curves do not generally correspond to significant changes in surface structure. Initial dipole moments calculated from the work function measurements are 7.4 ± 0.5, 7.2 ± 0.5 and 3.2 ± 0.3 Debye units for Na on Ni(111), (100) and (110), respectively, and 3.2 ± 0.3, 5.3 ± 0.3 and 7.0 ±0.5 Debye units for Na, K and Cs on Ni(110), respectively. A quantum mechanical theory developed by Gadzuk predicts dipole moments in good agreement with these experimental values for alkali metal coated Ni. The thermal desorption energies corresponding to the initial stage of adsorption measured for Na on Ni(111), (100), and (110) were 2.54, 2.52 and 2.18 ± 0.1 eV, respectively, and 2.18, 2.56 and 2.72 ± 0.1 eV for Na, K and Cs on Ni (110), respectively. The shapes of the desorption energy and frequency factor versus coverage curves were related to the surface crystallography. Finally, correlation of the desorption energy with the work function measurements emphasized the importance of ionic bonding at low coverages.

Structure analysis of alkali metal adsorption on single crystal nickel surfaces

Abstract Studies of alkali metal adsorption on single crystal Ni surfaces were undertaken to determine surface crystallography, electronic structure and atomic binding in these adsorption systems as a function of absolute surface coverage. A detailed analysis of surface crystallography using low energy electron diffraction (LEED) is presented and discussed in this paper. The work function and thermal desorption energy studies will be presented elsewhere. Significant information concerning the nature of surface forces was deduced from the surface crystallography. It is shown that alkali metal adatoms on Ni repel one another at all coverages forming a single layer. Furthermore, the forces between alkali metal adatoms on Ni (110) are highly anisotropic. These non-classical anisotropic forces are probably related to the indirect inter-adsorbate interactions via the metals conduction electrons treated by Grimley 6 ). The strong repulsive forces between adsorbed alkali metal atoms on Ni (111) and Ni (100) resulted in the adatoms being uniformly spaced . By uniformly spaced, it is meant that the standard deviation of the average distance between nearest neighbor adatoms is small. Incoherent hexagonal close-packed surface structures are demonstrated to form with Na on Ni (111) and Ni (110) at high coverages in order to maximize the packing of adatoms. The distance between nearest neighbor adatoms in these structures decreases continuously with coverage. The existence of these incoherent structures shows that the repulsive forces among alkali metal adatoms on Ni (111) and Ni (110) are more important in determining their positions at high coverages than are the variations in potential of the adatom due to substrate surface geometry. The distance between nearest neighbor alkali metal atoms adsorbed on Ni at one physical monolayer coverage is shown to be less than the diameter of bulk alkali metal atoms. The occurrence of a reduced alkali adatom diameter coupled with the existence of incoherent, close-packed surface structures raises significant uncertainty about the validity of conventional calculations of surface roughness factors from the density of adsorbed alkali metal atoms.

A quantum-mechanical model for the ionization and excitation of atoms during sputtering

A quantum-mechanical model is developed for the process by which an atom is excited or ionized as it is sputtered from a metal surface. The probability of excitation is given by R = (A/ΔE)2(hv/aΔE)n, where A is the binding energy of a surface atom before sputtering, v is its average velocity after sputtering, a is the thickness of the surface, and Δ E the excitation energy. For ionization, ΔE = I−φ, with I the ionization energy of the sputtered atom, and φ the work function of tke surface. Available experimental data for ionization are fitted best with a = (1.4 ± 0.3)A˚, and n = 2.5 ± 0.3. The model is expected to be applicable to bombarding energies up to about 100 keV.

Oxygen chemisorption and reaction on α-Fe(100) using photoemission and low-energy electron diffraction

Abstract The combined techniques of ultraviolet photoemission spectroscopy and LEED-AES have been applied to the study of the electronic and atomic structure of the Fe(100) surface during initial stages of chemisorption and oxidation. Prior to the occurrence of oxide nucleation, a half-monolayer of chemisorbed oxygen is formed, characterized by a single O(2p) peak in the UPS spectra and a C(2 × 2)LEED pattern. Valence electronic changes reflect the formation of FeO iron oxide at greater coverage. The oxide atomic structure is initially p(1 × 1) changing into superstructures associated with epitaxial oxide formation. A multielectron satellite peak is identified at ~ 8 eV separation from the iron d-band peak. Various shake-up mechanisms are discussed to interpret this as well as the observed splitting in the Auger peak at low energy.

Chemisorption and reaction of NH3 on Ni(111)

Abstract The chemisorption of ammonia on Ni(111) has been investigated using LEED, thermal desorption, and angle-resolved photoemission. For exposures at 200 K, thermal desorption shows a coverage-dependent binding energy associated with dipole-dipole interactions. A (2 × 2) LEED pattern occurs at 2–4 L exposure. Time dependence of the LEED pattern and changes in the thermal desorption induced by the LEED beam indicate that the (2 × 2) pattern is due to a stable intermediate decomposition species. Using synchrotron radiation photoemission all three valence orbitals of ammonia have been observed for the first time. The energies of the ammonia-induced features in the photoemission (−22.0, −11.0 and −6.7 eV below the Fermi energy) and the observed symmatries positively identify the absorbed species as molecular ammonia. Additional structure observed in the photoemission spectra after electron bombardment is associated with the stable adsorbed intermediate.

Nucleation of metal crystals on ionic surfaces

Abstract A systematic experimental study of the kinetics of nucleation of gold vapor onto the (100) cleavage face of magnesium oxide was made over the temperature range from 209–381°C in ultra high vacuum. The degree to which preferred nucleation occurred was found to depend on the defect structure of the surface. The experimental results are in quantitative agreement with a nucleation model originally developed for a defect-free surface and extended in this study to account for the contribution of point defects.

Leed investigations of xenon single crystal films and their use in studying the Ir(100) surface

Abstract Well-ordered, chemically pure, xenon crystals of the (111) surface orientation were grown on Ir (100) 1 × 1 and 1 × 5 type substrates at 55°K. Because of the strongly kinematic character of the xenon LEED intensity-energy spectra, it was possible to use the scattering spectra from adsorbed xenon to investigate the surface structures of Ir (100). The combination of information from the Auger spectra of the iridium, from the differences in orientation of the xenon crystals grown on the two Ir (100) structures, and from the positions of the Bragg-type interlayer scattering peaks in the adsorbed xenon-Ir (100) intensityenergy spectra, indicated in a self-consistent manner that the occurrence of the thermally stable 1 × 5 structure is associated with no identifiable surface impurity whereas it is consistent with the presence of a distorted hexagonal overlayer of iridium on a (100) plane typical of the fcc crystal.

Effect of surface deactivation on molecular chemisorption: Co on αFe(100) surfaces☆

Abstract Clean and partially deactivated αFe(100) surfaces show variable chemical reactivity towards carbon monoxide and unsaturated hydrocarbon molecules depending on preparation. Bonding of chemisorbed CO to clean Fe occurs according to the Blyholder model. UV-photoelectron spectroscopy (UPS) indicates a severe stretching of the C-O bond at low temperatures (98°K – 223°K) and complete dissociation at higher temperatures (≈ 300°K). Electron bonding interactions can be quenched to varying degrees by prechemisorption of C, O, P, or S. UPS spectra for CO chemisorption on Fe (100) + S indicate an unstretched molecule and provide a clear resolution of the 1π and 5σ levels due to reduced forward- and back-bonding interactions. These observations are directly related to the mechanism of sulfur poisoning in Fischer-Tropsch catalysis of higher hydrocarbons. The general utility of deactivation techniques for interpretation of surface-molecule interactions is discussed.