Robert L. Park

Soft x-ray appearance potential spectroscopy

The total soft x-ray emission of an electron-bombarded surface exhibits abrupt changes at the threshold potentials for the excitation of core levels of surface atoms. These features represent the excitation probabilities of core states superimposed on a smoothly increasing bremsstrahlung background. The threshold potentials can be sensitively detected by differentiating the current from an x-ray photocathode with respect to sample potential. This technique represents the highest resolution core-level spectroscopy available. We discuss the principles underlying the design and operation of soft x-ray appearance potential spectrometers and review their application to the study of surface composition, electron binding energies, chemical shifts, and band structure.

Flotation related ESCA studies on PbS surfaces

Abstract The oxidation of galena is known to play an important role in the beneficiation of lead sulfide by xanthate flotation. We have used electron spectroscopy for chemical analysis (ESCA) to characterize the nature of chemical species on PbS surfaces exposed to air and aqueous environments. PbS samples were in the form of thin sublimed films, natural galena crystals, and commercially available powdered material. Based on these photoelectron measurements, it may be concluded that lead sulfate is the major product of oxidation of PbS in air and aqueous environments. The presence of small amounts of elemental sulfur in the early stages of oxidation suggests that the initial oxidation step may differ from the major oxidation process. Galena occurs naturally in a wide range of stoichiometries. Oxidation was initially much slower in lead rich samples. In fact, the induction period exceeds the initial conditioning period in a typical separation plant. ESCA spectra of lead-rich sulfides confirms that the first step is the oxidation of Pb to PbO. Evidence is presented indicating that the interaction between lead sulfate and xanthates may be controlled by the semiconducting properties of the material.

Two‐dimensional chemisorbed phases

Chemisorbed atoms on single‐crystal metal surfaces frequently form commensurate diperiodic structures because of the strong, site‐specific binding. Since the adatoms are well described by the lattice gas model, these systems are valuable testing grounds for recent theories of two‐dimensional critical phenomena. We review briefly the developments in this field, emphasizing our results for the system O/Ni(111). This study includes an analysis of the multicritical low‐coverage phase diagram, Monte Carlo simulations of the overlayer, and the first measurement of critical exponents for a phase transition in a chemisorbed overlayer probed via low energy electron diffraction. We compare our results with predictions of various two‐dimensional models.

Quantum size effect in electron transmission through Cu and Ag films on W(110)

Abstract The transmission coefficient of very low energy electrons ( ⩽ 10 eV) normally incident on (111) epitaxial films of Cu and Ag on W(110) is modulated by interference between scattering from the vacuum/metal and metal/metal interfaces. Comparison with calculations of free-electron scattering from a one-dimensional potential model, in which grading of the metal/metal interface is represented by a smoothing of the potential step, indicates that this interface is abrupt within approximately one layer spacing. We obtain a value of 11.0 (8.0) ± 1.0 eV for the inner potential of Cu (Ag) and mean free path lengths of 39 ± 8 A at an energy of 7.0 eV relative to the Fermi energy and 29 ± 11 A at 9.0 eV for Cu, and 25 ± 10 A at 7.5 eV for Ag. Work function values are obtained by the field emission retarding potential technique. We investigate the effects of the surface potential barrier, inelastic scattering and surface roughness, and evaluate the validity of the one-dimensional model presented.

The Electronic Structure of Solid Surfaces: Core Level Excitation Techniques

The local density of states of atoms at solid surfaces can only be examined indirectly by techniques which use the core levels of the atoms as windows. The core levels themselves are measurably shifted by changes in the distribution of valence electrons. These “chemical” shifts are frequently measured by x-ray photoelectron spectroscopy (XPS), but more specific information on the local density of states is provided by techniques based on excitation by electron bombardment. Ionization spectroscopy (IS) extracts excitation edges from the total secondary emission spectrum. The threshold core level excitation probabilities can be obtained by soft x-ray characteristic isochromat spectroscopy (CIS), or more simply by soft x-ray appearance potential spectroscopy (SXAPS). Auger electron appearance potential spectroscopy (AEAPS) provides similar information. In each of these techniques the shape of the excitation edges is related to the density of conduction band states, but may be modified by final state interact...

Phase diagram of oxygen on Ni(100)

Abstract We have studied the coverage-temperature phase diagram of chemisorbed oxygen on Ni(100) with LEED and AES. We find that the oxygen p(2×2) structure undergoes a nearly reversible order- disorder phase transition for coverages between 0.15 and 0.30 monolayer ( T c ≈ 560 K for Φ = 0.25). For these coverages, it is also found that the chemisorbed oxygen begins dissolving into the bulk nickel as the overlayer disorders. A reversible order-disorder transition is not seen for the oxygen c(2×2) structure, but there is evidence that bulk dissolution here also occurs coincident with the overlayer disordering.

Effect of Steps on Low-Energy Electron Diffraction Intensity Profiles

A method is described for calculating the electron-solid elastic scattering amplitude when the surface area covered by the coherence width of the electron beam contains a relatively small number of steps. From this it is possible to calculate in detail the angular distribution of the scattered intensity for low-energy electron diffraction. In analogy with the perfect surface case, intensity profiles are defined which characterize the variation with primary beam energy of the central intensities for the various beams. It is shown that a step distribution reduces the central beam intensities with respect to the perfect surface case and also acts to shift the Bragg peaks in the intensity profiles to higher energies. Model calculations for simple step distributions are compared with Jonas experimental data for Al(110). For many adsorbed overlayer systems the problem of domains is simply a special case of our treatment

Anomalous Fine Structure in the Soft X-Ray Appearance Potentials of Nonmetals

A simple, nondispersive spectrometer has been used to study the binding energies of core electrons at solid surfaces. The spectrometer consists of a photomultiplier which measures the soft x-ray fluorescence of an electron-bombarded surface. The derivative of the multiplier current as a function of target potential rises abruptly at the threshold energies at which core electrons can be excited to unoccupied states. This produces a comparatively simple spectrum for metals with each level being represented by a sharp peak, followed by an abrupt step. For nonmetals, however, the spectrum of a single level can consist of a series of sharp peaks spread over as much as 30 eV.

Chromium Depletion of Vacuum Annealed Stainless Steel Surfaces

The susceptibility of vacuum annealed stainless steels to corrosion suggests a surface composition which is depleted in chromium. Direct confirmation of this has been obtained by soft x-ray appearance potential spectroscopy using a multiple sample arrangement in which samples of Cr, Fe, Ni, and 304 stainless steel were analyzed under identical conditions. This provides a reasonable calibration for the determination of surface concentration ratios only if the band structure of the constituents are nearly unaffected by alloying. Experimental evidence that this is approximately so for 304 stainless steel is presented. Using this calibration, the surface composition was determined as a function of isochronal annealing.

Formation of iron silicide thin films

The reaction of thin films of Fe with a silicon substrate to form iron silicides has been studied using Auger electron spectroscopy (AES) and electron energy loss spectroscopy. The formation of FeSi and FeSi2 is confirmed by comparison of plasmon losses with those reported for the bulk silicides. Changes in the Fe‐AES spectrum upon silicide formation result primarily from changes in the plasmon losses. Changes in the AES intensity are correlated with the formation of the two silicide phases. FeSi forms during heating around 400 °C and is stable at temperatures up to 530 °C. Further heating results in the formation of FeSi2 at around 550 °C. This phase is stable up to 670 °C.