A total-electron-yield technique is described in which near-surface extended x-ray-absorption fine-structure (EXAFS) data are obtained from direct measurements of specimen current. Experiments with several model systems---amorphous germanium and crystalline germanium, nickel, and cobalt; and arsenic ion implanted into silicon---demonstrate that this technique can reproduce EXAFS \ensuremath{\chi}(k) functions obtained from transmission and fluorescence measurements. Experiments also reveal that EXAFS amplitudes from total-electron-yield data can be 5--10 % smaller than those from transmission measurements for samples where the very-near-surface structure, at depths of tens to hundreds of angstroms, differs from the bulk structure. Measurements with buried layers confirm that the sampling depth for this total-electron-yield technique is determined primarily by the penetration ranges of Auger electrons emitted from the absorbing atoms. For the model systems listed above, LMM Auger electrons have ranges of hundreds of angstroms and KLL Auger electrons have ranges of thousands of angstroms. Expressions are derived for the sampling depth for total-electron-yield EXAFS experiments. The total-electron-yield technique described here is particularly useful for studying impurities within a few thousand angstroms of the surface of single crystals, where Bragg diffraction complicates the use of fluorescence measurements.

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Total-electron-yield current measurements for near-surface extended x-ray-absorption fine structure.

The phase and composition depth distributions of a low‐energy (0.7 keV), high‐flux (2.5 mA/cm2) N implanted fcc AISI 304 stainless steel held at 400 °C have been investigated by step‐wise Ar+ beam sputter removal in conjunction with conversion electron Mossbauer spectroscopy and x‐ray diffraction (XRD). A metastable, fcc, high‐N phase (γN), with both magnetic and paramagnetic characteristics, was found to be distributed in the N implanted layer generated by the low‐energy, elevated temperature, implantation conditions. The magnetic γN was found to be ferromagnetic and was distributed in the highest N concentration region of the implanted layer (the top 0.5 μm) while the paramagnetic γN becomes predominant below 0.5 μm, where the N content is only slightly lower. The ferromagnetic state is linked to large lattice expansions due to high N contents (∼30 at.%) as determined by XRD and electron microprobe. The relatively uniform XRD N distribution to a depth of ∼1 μm suggests a sensitive dependence of the magnetic γN phase stability on N concentration and degree of lattice expansion. The XRD results also show that the N contents and depths depend on the polycrystalline grain orientation relative to the ion beam direction. The N was found to diffuse deeper in the (200) oriented polycrystalline grains compared to the (111) oriented grains and the N contents were significantly higher in the (200) planes relative to the (111) planes. The effect of compressive residual stresses (∼2 GPa) is considered. The scanning electron microscopy (SEM) analysis reveals quite clearly the uniform nature of the γN layers with a reasonably well defined interface between the γN layer and the substrate, suggesting uniform N contents with uniform layer thicknesses within a given grain. However, they also show significant variations in the γN layer thickness from one grain to the next along the N implanted layer, clearly supporting the XRD findings of the variation in N diffusivity with grain orientation.

Phase and composition depth distribution analyses of low energy, high flux N implanted stainless steel

Inelastic electron scattering is calculated by means of a simplified model of the generalised oscillator strength density distribution of an atomic or solid target. The parameters of the model are a limited set of resonance energies (partial mean ionisation potentials) and oscillator strengths. Methods to estimate these parameters, involving a fit of the partial mean ionisation potentials to the empirical atomic mean ionisation potential, are described. The valence electron partial mean ionisation potential may be determined from the dielectric energy loss function, the local plasma approximation, or empirically. Good agreement with experiment and with recent calculations of stopping power and inelastic mean free path is obtained in the electron energy range 50 eV-50 keV.

https://iopscience.iop.org/article/10.1088/0022-3727/16/8/023/pdf

A simple calculation of inelastic mean free path and stopping power for 50 eV-50 keV electrons in solids

Publisher Summary This chapter discusses the use of Mossbauer spectroscopy for the study of problems in heterogeneous catalysis. The chapter describes the ways in which Mossbauer spectroscopy can be directly extended to “Mossbauer atoms or isotopes,” such as antimony, europium, nickel, ruthenium, gold, and tungsten. Using Mossbauer spectroscopy it is shown that lead enters the catalyst structure in the tetrahedral sites and results in an expansion of all tetrahedral sites and a contraction of the octahedral sites. The chemical promoter, lead, changes the electronic and geometric structure of the catalyst. A study of the different Mossbauer parameters and their temperature dependences can give information about the location of a non-Mossbauer isotope, lead, in its surrounding structure.

Mössbauer Spectroscopy Applications to Heterogeneous Catalysis

Transmission techniques are the dominating detection techniques in Mossbauer spectroscopy. As illustrated in Fig.1 the usual set-up consists of a (moving) source emitting Mossbauer γ-radiation which is partially absorbed in a resonance absorber; the transmitted radiation is detected in a suitable γ-detector as a function of a relative velocity between source and absorber. A typical transmission-spectrum shows a resonance absorption effect of a few percent. In general this percentage cannot be increased due to Debye-Waller factors f 1) effect-to-background ratios are obtainable for many cases. This advantage, however, is restricted by a loss in intensity due to a comparably small solid angle for the detection of the scattered radiation in conventional experimental arrangements.

T. Ekdahl

Papers

Analysis of the electron transport in conversion electron Mössbauer spectroscopy (CEMS)

Depth selective Mössbauer-effect measurements by means of scattered electrons

An electrostatic spectrometer for conversion electron Mössbauer spectroscopy

Depth selection by means of scattered electrons: a method to determine electron line profiles

A simple method for the analysis of depth-selective Mössbauer-effect measurements