TABLE OF CONTENTS

1 Physical Concepts of the Method 1
1.1 General Aspects of Mössbauer Spectroscopy 1
1.2 Hyperfine Interactions and Line Positions in Mössbauer Spectra 5
1.3 Relative Intensities of Spectral Lines 17
1.4 Experimental 24
References 30

2 Mössbauer Spectroscopy Based on Detection of Electromagnetic Radiation 32
2.1 Radiation Transmission Through Matter 32
2.2 Low Energy  Quanta Scattering 49
2.3 Resonance Fluorescence and Interference Effects 55
2.4 Angular Dependences of the Scattered  Radiation 74
2.5 Mössbauer  Quanta Scattering as a Method of Surface Study 78
2.6 Scattering Experiments with Detection of Characteristic X rays 101
2.7 A Theory of Backscattering Mössbauer Spectroscopy (X ray detection) 107
2.8 Backscattering Mössbauer Spectroscopy by the Detection of X and  Radiation. Practical Aspects 124
References 131

3 Mössbauer Spectroscopy Based on the Detection of Electrons 135
3.1 The Interaction of Electrons with Matter Following Mössbauer Scattering 136
3.2 Conversion Electron Mössbauer Spectroscopy. Theory Based on the Exponential Attenuation of the Electron Beam 148
3.3 Theory of CEMS Based on Elementary Electron Interactions 159
3.4 Depth Selective Conversion Electron Mössbauer Spectroscopy 174
3.5  Spectra and Weight Functions for DCEMS 188
3.6 Structure Determination of Near Surface Layers by DCEMS 206
3.7 Proportional Counters as Electron Detectors in CEMS 220
3.8 Depth Selection by Means of Proportional Counters 230
3.9 Analysis of Thin Layers under Total External Reflection of Mössbauer Radiation 248
3.10 Other Methods of Detection in CEMS 257
3.11 Channeltrons and the Detection of Very Low Energy Electrons 262
References 269

4 Mössbauer Backscattering Spectra 278
4.1 Weight Functions and Re scattering 279
4.2 Evaluation of the Intensity of Scattered  Radiation 285
4.3 Evaluation of the Intensity of X rays and Electrons 293
4.4 Line Shapes and Intensity Ratios 302
4.5 The Quality of a Mössbauer Spectrum 310
4.6 Quantitative Information from Mössbauer Spectra 315
4.7 Layer by layer Analysis
References 330

5 Practical Aspects of Surface Layer Analysis 333
5.1 Corrosion Studies of Metals and Alloys 333
5.2 Applications in Metal Physics 362
5.3 Ion Implantation and Laser Treatment of Metals 378
5.4 Spin Texture Studies of Surface Layers 396
5.5 Some Other Applications 409
References 420
Variables and Abbreviations (alphabetic) 435
Index 453

PREFACE

The study of solid surfaces is one of the most important problems in solid state science. One feature of such studies is the development of techniques which are able to examine solids over a wide range of compositions, gaseous pressures, different temperatures and under conditions of practical interest. The surface of such specimens of interest may be of any shape. It is also important to note that the different fields of science and technology have different notions of what constitutes a surface. For example, a physicist dealing with the interaction of absorbed atoms with the substrate surface means by the latter the actual geometric surface or a layer with the depth of less than 1 nm. A metallurgist's interest in a surface is generally that region of the solid which is tens and hundreds of m deep. This is one of the reasons why some physics encyclopedia give no definition of a surface.
At this stage it is relevant to note that emission Mössbauer spectroscopy is capable of the examination of 1012 atoms implanted on a square centimetre which is equivalent to one hundredth of the number of atoms making up a monolayer on the area of one square centimeter. It is important to note that the technique can investigate the properties of layers which are tens of m deep. Whilst nuclear physics has developed many novel methods of surface modification (e.g. implantation) and effective nuclear research methods, Mössbauer spectroscopy has developed as one of the few methods available for investigation of solids differing in depth by several orders of magnitude. Hence, it is only in recent times that the problems of surface investigation and the study of separate layers have been amenable to investigation.
Two factors may be identified which are responsible for the widespread use of Mössbauer spectroscopy in both fundamental and applied surface science research. Firstly, the absolute selectivity of Mössbauer spectroscopy which means that in each experiment a response is registered from only one isotope of the element. Furthermore, atoms of that element in non equivalent positions give different responses with magnitude which is proportional to the populations of the positions. Secondly, Mössbauer spectroscopy has a high sensitivity which is determined by the minimum number of resonant atoms needed to get a detectable response. In the transmission Mössbauer spectroscopy for 57Fe a response is given by a monolayer with the area of the order of 1 square centimetre. However, as has already been mentioned, higher sensitivities can be achieved in emission Mössbauer spectroscopy.
The parameters of the hyperfine interaction derived from the Mössbauer spectra provide valuable information on the chemical bond character and on magnetic properties of surface layers as well as on the change of the properties with the depth from the outermost surface layer. It is possible to carry out quantitative phase analysis and to use the technique to study different transformations in the solid which result from external effects under a wide range of temperatures and pressures. Certain problems are sometimes encountered which are caused by the limited activities of Mössbauer sources and by the rather low cross section of the radiation interaction with matter. However, such problems are compensated by the absence of any limitations on experimental conditions other than that the substance should be a solid. As a consequence, Mössbauer spectroscopy can be used in fundamental research and in various areas of applied science and technology including process monitoring. This book is one of the first attempts at a consistent presentation of theoretical and practical problems of the use of Mössbauer spectroscopy to study solid surfaces, its applications, and development.
The applications include: surface studies with hyperfine probes in the following fields: oxidation and corrosion of metals and alloys; passivating and protective coatings; physics of metals: annealing and quenching, mechanical and chemical treatment, ion implantation and laser treatment; texture of near surface layers. The choice is based on scientific interests of the author and his practical experience in these fields. However, the limited space does not allow complete coverage of any of the topics mentioned above. It is also important to note some other fields for example the unique capabilities of Mössbauer spectroscopy for fine particles studies: superparamagnetism, phase analysis and the magnetic structure of the surfaces of particles which may be smaller than 10 nm; the effect of the gas phase on the properties of small particles, the interaction of these particles with the substrate; and the importance of these studies in areas of industrial significance such as catalysis. It should also be noted that Mössbauer spectroscopy is one of the best methods for in situ characterization of solid/solid and solid/solution interfaces. This lends itself to in situ studies of surfaces under various coatings and processes, surface magnetism and the effect of the gas phase on the properties of the surface layers and the structure and magnetic properties of epitaxially grown monolayers on the surface of oriented single crystals .
I wish to thank all the colleagues whose advice made it possible for the final version of the book to be published. I am especially indebted to Professors R.N. Kuzmin, B.S. Pavlov, G.V. Smirnov and Drs A. Dozorov, A. Ryzhkov and V.G. Semenov from Russia, Prof. Ph. Gütlich and Dr. W. Meisel from Mainz University, Prof. J. G. Stevens, Mössbauer Effect Data Center, University of North Carolina, Prof. H. De Waard from the Netherlands and Dr. F. Berry from University of Birmingham (England).