Positron Annihilation in Chemistry

278 Pages·1995·10.38 MB·English

by  Dr. Ole Erik Mogensen (auth.)

58
Springer Series in Chemical Physics
Edited by V.I. Goldanskii
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Springer Series in Chemical Physics
Editors: Vitalii I. Goldanskii Fritz P. Schafer 1. Peter Toennies
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Ole Erik Mogensen
Positron Annihilation
in Chemistry
With 44 Figures and 13 Tables
Springer-Verlag
Berlin Heidelberg New York London Paris
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Dr. O.E. MOGENSEN (t)
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Max-Planck-Institut
fiir Biophysikalische Chemie
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Preface
The electron, positron, and positronium (Ps, the electron-positron bound state)
are the only light particles which participate in normal low-energy processes
of physics and chemistry. Because of their small masses quantum mechanical
phenomena play a great role in their behavior. Good accounts of the excess
electron behavior are available. Similarly, several good books treat the behavior
of the positron in solid state and gas physics. However, the research on the
positron and Ps in chemistry-oriented fields of science is not well treated in
the literature. In liquids Ps is normally in the "bubble" state, and hence, its
chemical reactions are very exceptional, indeed. The states and reactions of the
positron in insulating liquids can profitably be compared to those of the only
other charged light particle, the excess electron. Furthermore, Ps is formed
by a reaction between the positron and excess electron in the positron spur,
and Ps formation studies give, therefore, information on excess electrons, too.
Consequently, to understand "light particle chemistry" it is necessary to have a
good understanding of the chemistry of the positron and Ps.
This book is a critical review of the research on the behavior of the positron
and Ps performed within chemistry-oriented positron annihilation. In particular,
I have emphasized the interpretation of selected, typical, experimental results
in the field. The book does not give an exhaustive discussion of all the pub
lished results. This is impossible because of lack of space anyway. Discussions
of experimental results and theoretical explanations believed to be outdated,
erratic, or misleading are mainly omitted, with the exceptions of those which
have been used extensively in the literature. The omission of certain works
might just indicate that the accepted ideas and concepts in the field have changed
very much recently.
Partly because of lack of space positron annihilation in several fields on the
borderline of chemistry, physics, and/or materials science is not discussed. For
example, research on metals, semiconductors, ionic crystals, ceramic materials,
high Tc superconductors, gases, and liquid noble gases is not discussed, except
if the results are of general interest.
The writing of this book would have been impossible without many years
of very good collaboration with M. Eldrup and N.J. Pedersen at Risoe. Further
more, I spent seven months doing fruitful work at IMFUFA, Roskilde University
VI Preface
Center, and I have been finishing the manuscript during my stay at the Chemistry
Department, JAERI, Japan. I am very thankful to both these institutions.
Tokai-mura, Japan, December 1991 O.E. Mogensen
Sadly, the author of the present book, Ole Erik Mogensen, suddenly passed
away in April 1993. He therefore never saw this book in which he had invested
so much effort.
The editorial procedure was in progress at the time of his demise, and
the reviewer's comments on the manuscript were not in time for him to con
sider them. The book, as it now appears, is thus the original version from Ole
Mogensen's hand apart from the corrections of some misprints and an appended
subject index.
RiSf/!, August 1994 M. Eldrup
Contents
1 Introduction...................................... 1
1.1 Positron Annihilation in Chemistry . . . . . . . . . . . . . . . . . . 1
1.2 History of Chemistry-Oriented Positron
Annihilation Studies .. . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Review of Physics-Oriented Positron Research ......... 9
1.4 The Content and Intention of this Book .............. 12
2 Theory.......................................... 15
2.1 The Annihilation Probability ...................... 15
2.2 Atomic Physics of Positronium (Ps) . . . . . . . . . . . . . . . . . 21
2.3 Application of the Theory . . . . . . . . . . . . . . . . . . . . . . . . 25
3 Experimental Techniques ............................ 29
3.1 Positron Lifetime (LT) Technique. . . . . . . . . . . . . . . . . . . 29
3.1.1 Elementary Description of a LT Spectrometer . . . . . 30
3.1.2 Special Problems of LT Measurements. . . . . . . . . . 32
3.1.3 General Remarks on the Data Analysis. . . . . . . . . . 35
3.1.4 Data Analysis of LT Spectra. . . . . . . . . . . . . . . . . 36
3.2 Angular Correlation (AC) Measurements. . . . . . . . . . . . . . 37
3.2.1 Experimental............................ 37
3.2.2 Data Analysis of AC Spectra. . . . . . . . . . . . . . . . . 41
3.3 Doppler Broadening (DB) Measurements ............. 42
3.4 Miscellaneous Measurements. . . . . . . . . . . . . . . . . . . . . . 44
4 Light Particle States in Insulating Matter . . . . . . . . . . . . . . . . 48
4.1 General Remarks .............................. 48
4.2 The Excess Electron . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.3 The Positron States in Liquids. . . . . . . . . . . . . . . . . . . . . 53
4.4 The Ps States in Liquids . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.5 Short Remarks on Light Particle States
in Solids and Gases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.6 Measurable Properties of Light Particles . . . . . . . . . . . . . . 63
VIII Contents
5 The Positron Spur and Ps Formation in Pure Liquids . . . . . . . 66
5.1 The Positron Track and its Terminal Spur. . . . . . . . . . . . . 66
5.2 Homogeneous, Long Time Irradiation
by Positron Slowing Down ....................... 69
5.3 Determination of the Ps Yield ..................... 71
5.4 Formation of Ps in the Positron Spur ................ 77
5.5 The Measured Ps Yields for Pure Liquids. . . . . . . . . . . . . 80
6 Solvated Positron Reactions. Models of Reaction Kinetics . . . . 88
6.1 Hydrated Positron Reactions with Halides . . . . . . . . . . . . . 89
6.2 Reaction Kinetics in Positron Annihilation ............ 98
6.3 Discussion of e~ Reactions. Low Concentrations. . . . . . .. 106
6.4 Discussion of t!a~ Reactions. High Concentrations ....... 110
7 Positronium Formation in Solutions .................... 112
7.1 Special Cases of Ps Formation in Nonpolar Mixtures. . . .. 112
7.2 Theory of Ps Inhibition and Antiinhibition ............ 116
7.3 Empirical Expression for the Ps Yield. . . . . . . . . . . . . . .. 118
7.4 Positronium Formation in Mixtures of Nonpolar Liquids .. 120
7.5 Positronium Formation in Solutions of Polar Liquids . . . .. 132
7.6 Miscellaneous Ps Yield Effects. . . . . . . . . . . . . . . . . . . .. 146
7.7 The Older Ps Formation Models. . . . . . . . . . . . . . . . . . .. 150
8 Positronium Reactions in Solutions. . . . . . . . . . . . . . . . . . . .. 155
8.1 Kinetic Equations of Positronium Reactions ........... 155
8.2 An Example of Ps Quenching and Inhibition . . . . . . . . . .. 162
8.3 Positronium Quenching by Mainly Organic Molecules
in Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 165
8.4 Paramagnetic Quenching of Positronium in Solutions. . . .. 176
8.5 Positronium Reactions with Ions in Aqueous Solutions. . .. 183
9 Principles of Positron Annihilation in Molecular Solids . . . . .. 193
9.1 Positronium Bloch Functions and Ps Trapping in Defects .. 194
9.2 Positronium Trapping in Vacancies in Plastic Crystals .... 200
9.3 Positronium Formation in Solid Aromatic Hydrocarbons. .. 206
9.4 Positronium in Some Other Molecular Crystals . . . . . . . .. 211
9.5 Slow-Positron Beam Studies of Solids ............... 217
10 Polymers and Miscellaneous Molecular Compounds ........ 221
10.1 Transition from Glassy to Normal Liquid State . . . . . . . .. 221
10.2 Positron Annihilation in Polymers .................. 223
10.3 Electric Field Dependence of the Ps Yield. . . . . . . . . . . .. 229
Contents IX
lOA Liquid Crystals and Miscellaneous Liquids . . . . . . . . . . .. 234
10.5 Miscellaneous Solids . . . . . . . . . . . . . . . . . . . . . . . . . . .. 239
11 Conclusion....................................... 246
References .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 248
Index of Chemicals ........................................ 259
Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 265
1 Introduction
The chemical physics of the positron and positronium (Ps) is concerned with
the chemical aspects of the interaction of low-energy positrons with matter.
Positrons injected into matter thermalize rapidly. In many insulating compounds
some of the positrons form Ps. The positron and Ps might react with solutes,
etc. before they ultimately annihilate into gamma photons. The properties of
these photons, measured by gamma spectroscopy techniques, give information
about the positron and Ps reactions before the annihilation.
This chapter starts with a short general discussion of the basic principles
of the chemistry-oriented positron annihilation research. The next section gives
a brief description of the history of this subdivision of low-energy positron
annihilation. A short discussion of the physics-oriented disciplines of the field
is included in Sect. 1.3. In the last section a discussion of the content of this
book is given.
1.1 Positron Annihilation in Chemistry
The positron is the antiparticle of the electron. Hence, it has the same mass
as the electron, and its charge is numerically identical to, but of opposite sign
of that of the electron. Energetic positrons injected into condensed matter are
slowed down to thermal energy in a very short time of about one ps. They pen
etrate 10-1000 ~m into condensed matter. Thermalization occurs by creation of
excess-electron positive-ion pairs along the track. The last part of this radiation
damage along the track will take place at a short distance from the thermalized
positron. It is called the positron spur.
In many insulating solids and liquids some of the injected positrons form
positronium (Ps) with an electron of the sample. Positronium is a bound state
of an electron and a positron. It is the lightest known atom, which can formally
be obtained from a hydrogen atom by replacing the proton by the positron.
The thermalized positron cannot form Ps with one of the electrons bound in
the molecules, since the binding energy of an electron in Ps is only 6.8 eV
compared to that in ordinary molecules of 9-12 e V. However, Ps can be formed
by a reaction between the thermalized positron and one of the excess electrons
in the positron spur. This occurs in competition with other spur processes,