Soft X-ray Appearance Potential Spectroscopy Research Articles

Soft X-ray appearance potential spectroscopy of O chemisorbed on Ti, Cr, Fe and Ni

We wish to report the observation of O 1 s level soft X-ray appearance potential spectra for O adsorbed on clean polycrystalline surfaces of Ti, Cr, Fe and Ni. The spectra are detectable in the chemisorption regime and are then characterized by one single peak at the threshold. The peak is thought to be related to an enhanced local density of unfilled valence states on the oxygen atoms. These simple spectra are in sharp contrast to the multipeaked O 1 s level spectra observed from the surface oxides grown after prolonged oxygen exposures.

Soft x-ray appearance potential spectroscopy of lanthanum, gadolinium, and thorium

The electron binding energies of La (M2–N 5,4 levels), Gd (M3–N 5,4 levels) and Th (O5–N1 levels) were determined by SXAPS and compared with those obtained by ESCA and by calculation. The results are in reasonable agreement. The La was oxidized and the shift in the SXAPS peaks was less than 1 eV. Satellite peaks have been observed associated with the M5, M4, and N 5,4 peaks of La and Gd and the O5 and O4 peaks of Th. Possible mechanisms responsible for these satellite peaks are discussed.

Dynamic background subtraction and the retrieval of threshold signals

An approach is outlined for handling the general experimental problem of retrieving characteristic signals from the presence of large backgrounds. The method consists of n th-order differentiation followed by multiple integration of the same order. It is shown that this procedure results in a function which has the form of the remainder of the (n - 1)th-order Taylor series for the background plus characteristic signals. If the function is one with a very large radius of convergence, then an order can be found such that the remainder is arbitrarily close to zero over the region of interest. Threshold functions, which are characterized by having very small convergence radii and which asymptotically approach zero on one or both sides of the point corresponding to the minimum radius of convergence, are shown to be accurately retrieved if the minimum convergence radius is contained in the region of integration. The particular advantages of the technique are demonstrated by its application to soft x-ray appearance potential spectroscopy.

A Soft X-ray Appearance Potential Spectroscopy Study of the Oxidation of Nickel

Abstract Variations of the electronic properties of nickel during the first stages of oxidation were studied by means of the soft x-ray appearance potential spectroscopy (APS). The spectra consist of a superposition of contributions from metallic nickel and from NiO and yield no evidence for a continuous change of the oxidation state of the Ni atoms. The binding energy of the 2p3/2-electrons in metallic nickel was found to be 851 eV and is increased in NiO by 1.4 eV. It is concluded from the shape of the APS peaks that with Ni above the Fermi level 3d- and 4s-bands are superimposed whereas with NiO these are separated in energy.

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.

X-Ray Filtering to Improve Signal-to-Noise in Soft X-Ray Appearance Potential Spectroscopy

A method is described which improves the signal-to-noise ratio in soft x-ray appearance potential spectroscopy (SXAPS). The incoming photon beam is filtered ahead of the detector by means of thin metallic foils or organic films, resulting in reduction of very low energy photons which produce unwanted noise in the detector. A specific example is given of the use of a 6.3 μ aluminum foil filter in the study of samarium M spectra.

Comment on ``Appearance potential spectroscopy''

Contrary to assertions by Tracy that soft x-ray appearance potential spectroscopy cannot be applied to semiconductors and insulators, we have encountered no obstacles in obtaining good spectra from both classes of materials. In addition to previously published spectra of boron and transition-metal oxides, we present 1s and 2p spectra of clean silicon.

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 interactions and core hole lifetime broadening. The lifetimes can be determined independently by XPS.

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.