Table Of ContentMonographs on Electron Spin Resonance
T.H. Wilmshurst
Electron Spin
Resonance
Spectrometers
ELECTRON SPIN RESONANCE SPECTROMETERS
MONOGRAPHS ON ELECTRON SPIN RESONANCE
Editor: H. M. Assenheim, Hilger & Watts Ltd, London
INTRODUCTION TO ELECTRON SPIN RESONANCE, H.M. Assenheim
ELECTRON SPIN RESONANCE IN SEMICONDUCTORS, G. Lancaster
ELECTRON SPI~ RESONANCE SPECTROMETERS, T.H. Wilmshurst
Electron Spin Resonance
Spectrometers
T. H. WILMSHURST
Univer,<ity of Southampton
Springer Science+Business Media, LLC
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ISBN 978-1-4899-5599-9 ISBN 978-1-4899-5597-5 (eBook)
DOI 10.1007/978-1-4899-5597-5
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T. H. WILMSHURST, 1967
Originally published by Plenum Press in 1967.
Softcover reprint of the hardcover 1s t edition 1967
CONTENTS
'l Fundamental requirements 1
2 Development of a basic spectrometer 17
3 Microwave systems 62
4 Spectrometer cavities 108
5 Superheterodyne spectrometers 142
6 Automatic frequency control systems 168
7 Low-noise microwave pre-amplifiers 217
8 The spectrometer magnet system 225
9 Electronic circuitry 234
Bibliography 267
Glossary 268
Index 277
v
EDITOR'S PREFACE
Dr Wilmshurst's book is intended for physicists and engineers
who wish to know more about the working principles of electron
spin resonance and its associated circuitry.
This volume covers all facets of E. S. R. instrumentation from
the simplest to the most complex spectrometer designs. It is thus
a complete guide to all forms of E. S. R. spectrometer, and will
enable many to design and construct their own systems from first
principles. Classical systems are critically discussed and the
author's conclusions embody the most up-to-date techniques and
circuitry. There is also very good coverage of the theory behind
all parts of the system. In particular, Chapter 4 discusses the
design of E.S.R. cavities, resonant and otherwise, so that the
non-specialist can now design and construct his own.
For the electronic engineer or microwave engineer who is not
concerned with E.S.R., Dr Wilmshurst presents between these
covers a complete survey of high-gain low-noise circuitry, A. F. C.
for microwave systems, methods of superheterodyne detection,
and a multitude of other techniques.
This book forms a most useful addition to the Hilger Mono
graphs on E.S.R., and will be invaluable to any E.S.R. spec
troscopist concerned with the practical aspects of his subject.
There has long been the need for such a treatment.
H. M. ASSENHEIM
vi
AUTHOR'S PREFACE
In writing this book my purpose has been to set out a systematic
approach to the design of an E.S.R. spectrometer. As electron
spin resonance is a branch of microwave spectroscopy, the de
sign problem historically has been ·a matter of translating into
terms of microwave hardware the ideas of other branches of spec
troscopy. The first chapter traces this development, which draws
from optical, infra-red, gaseous microwave and N.M.R. spec
troscopy, and also from the field of microwave engineering.
In subsequent chapters I have developed a circuit model for
the microwave system and derived an optimum arrangement.
Chapter 4 discusses in detail the microwave cavity and covers
the problem of wet samples. The remaining chapters are devoted
to superheterodyne spectrometers, A.F.C. systems, the magnet
and power supplies, and finally, the electronics.
I have assumed that the reader has a graduate knowledge of
electronics and microwaves; if he has not, I suggest that he
consults some of the books listed in the bibliography.
I am particularly grateful to Dr C.P. Poole, Jr, for allowing me
a preview of the breakdown of his book Experimental Techniques
in Electron Spin Resonance. This has enabled me to avoid
duplication, and ideally the present text should be read in con
junction with the other. Dr Poole has provided an excellent and
extremely comprehensive review of the various spectrometer
designs that have been used. Hence I have limited the scope of
my own book to a more detailed discussion of design principles.
Many of the designs described are used in commercial E.S.R.
spectrometers. However, as it would have been impractical to
list every instance where this is so, I have deliberately avoided
mentioning them. I am grateful for the help of a number of manu
facturers, who have suggested many of the ideas which I have
developed in this book. In particular, I should like to thank the
following people: Mr P. Butcher of the Decca Radar Company,
who allowed me to spend six weeks with his group at Hersham;'
Mr H.M. Assenheim of Hilger & Watts Ltd, who has given me full
access to the manuals of the Microspin spectrometer and with
whom I have had many valuable discussions; and Dr A. Horsfield
of Varian Associates, who has allowed me to see some of the
vii
instrumentation produced by his company. This is not a com
prehensive list of manufacturers, but simply includes those with
whom I have had personal discussions.
Thanks are also due to Professor D.J .E. Ingram for introducing
me to the subject, and to Mr L.G. Stoodley of R.M.C.S., for innu
merable valuable discussions over the past ten years. I am
particulary grateful to Mr Assenheim for his careful editorship,
stimulation and encouragement, and also to Mr David Tomlinson
for seeing the book through the press.
T.H. WILMSHURST
DEPARTMENT OF ELECTRONICS
UNIVERSITY OF SOUTHAMPTON
June, 1967
viii
Chapter 1
Fundamental Requirements
This chapter introduces the subject of E.S.R. spectrometers and
indicates their fundamental requirements. First, a brief preliminary
discussion of the methods of optical spectroscopy and microwave
absorption spectroscopy of gases is given, and then it is shown
how the techniques of E.S.R. spectroscopy have developed from
them. Enough will be said about the theory of the process of
E.S.R. absorption to provide a foundation for the quantitative
design of the spectrometer and to give some idea of what kind of
spectra to expect. The treatment is necessarily brief and is not
intended to be even an introduction to the general topic of E.S.R.
spectroscopy. When such an introduction is required it may be
found in the first volume1 in the present series or in other texts
listed in the bibliography.
§1.1
OPTICAL SPECTROSCOPY
Optical spectroscopy is the oldest branch of spectroscopy, origi
nating from the simple observation that materials absorb light of
some frequencies and transmit light of others. To observe optical
absorption spectra, all that has to be done is to pass a pencil of
light through the sample and then through a prism. The prism
refracts unequally the various frequency components constituting
white light, so that when the light falls on a screen different
frequency components appear at different points. Frequencies ab
sorbed by the sample produce dark bands or lines on the screen.
The frequency resolution of such a system depends on the width
of the pencil of light, and so a better arrangement for obtaining
good resolution is that of Fig. 1.1. Here, light from the source
passes through a narrow slit to a lens which collimates it into a
1
