S. M. Irkaev

Grazing incidence Mössbauer spectroscopy: new method for surface layers analysis: Part I. Instrumentation

Abstract The aims of this series of papers are to describe a spectrometer for simultaneous investigation of Mossbauer spectra from: (1) specularly reflected gamma-rays, (2) secondary electron, (3) characteristic X-ray, and (4) scattered gamma-rays (part I); to present a general theory of such spectra and to indicate some unusual characteristics and features of Mossbauer spectra at grazing angles (part II); and to give a quantitative analysis of experimental spectra for some 57 Fe films which shows that grazing incidence mossbauer spectroscopy (GIMS) is really a new method of surface investigation (part III).

Grazing incidence Mössbauer spectroscopy: new method for surface layers analysis: Part II. Theory of grazing incidence Mössbauer spectra

Abstract A general theory of grazing incidence Mossbauer spectroscopy (GIMS) spectra for an inhomogeneous layered medium is presented. The computer simulation based on this theory shows that the shape of the resonant spectrum of the reflected wave intensity, in the case of a resonant film, differs considerably from the case of a semiinfinite resonant mirror. This shape strongly depends on the thickness and the properties of the film and of the nonresonant substrate. The proposed theory of the lineshape of Mossbauer secondary radiation spectra from resonant films takes into account the influence of different types of photo- and conversion electrons on the resulting spectrum. It is shown that the fitting of grazing incidence Mossbauer spectra can be done only by means of numerical simulations.

SECONDARY RADIATION EMISSION AT MOSSBAUER TOTAL EXTERNAL REFLECTION

The effects of energy dependence of secondary radiation emission (SRE) followed by nuclear resonant and nonresonant photo-absorption at Mössbauer total external reflection (MTER) have been considered theoretically and checked experimentally. Numerical interpretation of a set of MTER and CEM spectra at different grazing angles has given the depth profiles of electronic density, photoabsorption coefficient and hyperfine interaction variations in 50 nm slightly oxidized57Fe film. Empirical functions of photo- and conversion electron yield are also determined. They appeared to be nonsimilar and nonmonotone. The energy dependencies of SRE are recalculated for time domain Mössbauer spectroscopy of synchrotron radiation (SR). The coherent nature of the TER effect reveals itself in the existence of delayed intensity of nonresonant SRE. The relation of nonresonant and resonant SRE, their energy, time and angular spectra strongly depend upon the depth of their creation, which opens a way for depth selective measurements as in the X-ray standing waves method.

Grazing incidence Mössbauer spectroscopy: a new method for surface layer analysis Part III. Interpretation of experimental data

Abstract Quantitative analysis of two sets of experimental GIMS data is presented. A computation procedure, which allows us to determine the distribution versus depth separately for each kind of hyperfine interaction in 57 Fe film for different states of oxidation is described. The result of our analysis shows that grazing incidence Mossbauer spectroscopy is really a new technique for surface investigation.

Mössbauer total external reflection: Composition of surface layers

An ambiguity of Mössbauer spectra measured under total external reflection conditions due to the increase of the number of parameters which should be determined can be compensated by measuring a series of experimental spectra. The interpretation of all experimental data must be done by means of numerical modeling (or fitting). The method enables us to study phase transformations within a layer of about 10 nm thickness.

Methodological problems of quantitative analysis in Mössbauer spectroscopy

The results from research into the effect of different factors on errors in quantitative determination of the phase composition of studied substances by Mössbauer spectroscopy ion absorption are presented, and ways of using them are suggested. The effectiveness of the suggested methods is verified by an example of analyzing standard and unknown compositions.

Highly sensitive Mössbauer spectrometer for SEDM and RSMR investigations

Abstract The performance, operation and functional capabilities of the spectrometer are described. The spectrometer is intended for Mossbauer investigations of dynamic processes in condensed media containing resonance isotope nuclei (selective excitation double Mossbauer effect, SEDM) or not (Rayleigh scattering of Mossbauer radiation, RSMR). The results of investigations on the optimization of gamma-optics of the spectrometer as illustrated by experiments on selective excitation are presented. The variety of these gamma-optics depends on the characteristics of the detectors employed for registration of scattered radiation. It is shown that introduction of resonance detectors increases the sensitivity, accuracy and reduces acquisition time by a factor of 10. The spectrometer is equipped with two Doppler modulation systems which can operate in both synchronous and asynchronous modes. The accuracy of adjustment to a required constant velocity value and of reproducibility is 0.04%. The spectrometer allows one to make a further step forward in the study of dynamic processes.

Determination of the oxidation state of iron by X-ray fluorescence spectroscopy using chemometric approaches

A possibility of identification of the oxidation state of iron by wavelength dispersive X-ray fluorescence spectroscopy using both the position and intensity of Lα and Lβ spectral lines of iron and by principal component analysis score data obtained by the decomposition of the spectral region corresponding to spectral L-series lines of iron is demonstrated. The application of scores ensures a more reliable identification in comparison with line parameters (position and intensity). Two approaches based on projection on latent structures (PLS) regression for the determination of the concentration of iron in different oxidation states are proposed. The first approach consists in using reference models with compositions similar to those of analyzed samples. In the second approach, PLS regression was build using model spectra obtained from spectra of readily available iron compounds.

Application of resonant detectors in Mössbauer spectroscopy

The application of resonant detectors in Mössbauer spectroscopy allows us to increase resolution and sensitivity and thus to expand the information potential of this method. In this work, the particulars of applying resonance detection are described and its advantages are demonstrated experimentally in comparison with conventional methods of detection.

Fluorescence analysis of a multilayer (Zr(10 nm)/[Fe(1.6 nm)/Cr(1.7 nm)]26/Cr(50 nm)/glass) structure under grazing incidence conditions

Element-specific measurements of the fluorescent radiation combined with measurements of the reflectivity curves have been performed for a Zr(10 nm)/[Fe(1.6 nm)/Cr(1.7 nm)]26/Cr(50 nm)/glass sample with the use of a new experimental setup created at the Institute for Analytical Instrumentation. The experimental data have been processed using a FLUO simulation program package implemented in the Visual C++ environment. The Zr, Fe, Cr, and Sn depth profiles have been reconstructed via coprocessing of reflectivity and fluorescence yield curves that were measured for each of these elements.