Title: Mössbauer studies of surface layers
Description: 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).
Subjects: Mössbauer spectroscopy, Surface chemistry