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Principles of
Field Ionization and
Field Desorption
Mass Spectrometry
by
H. D. BECKEY
PERGAMON PRESS
OXFORD • NEW YORK • TORONTO • SYDNEY • PARIS • FRANKFURT
U.K. Pergamon Press Ltd., Headington Hill Hall,
Oxford 0C3 OBW, England
U.S.A. Pergamon Press Inc., Maxwell House, Fairview Park,
Elmsford, New York 10523, U.S.A.
CANADA Pergamon of Canada Ltd., 75 The East Mall, Toronto,
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AUSTRALIA Pergamon Press (Rust.) Pty. Ltd., 19a Boundary Street,
Rushcutters Bay, N.S.W. 2011, Australia
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75240 Paris, Cedex 05, France
FEDERAL REPUBLIC Pergamon Press GmbH, 6242 Kronberg/Taunus,
OF GERMANY Pferdstrasse 1, Federal Republic of Germany
Copyright © 1977 H. D. Becket'
All Rights Reserved. No part of this publication may be reproduced, stored in a
retrieval system or transmitted in any form or by any means: electronic, electrostatic,
magnetic tape, mechanical, photocopying, recording or otherwise, without
permission in writing from the author.
First edition 1977
British Library Cataloguing in Publication Data
Becket', Hans-Dieter
Principles of field ionization and field desorption
mass spectrometry.
1. Mass spectrometry
I. Title
545'.33 QD96.13 77-30314
ISBN 0-08-020612-3
In order to make this volume available as economically and as rapidly as possible the
author's typescript has been reproduced in its original form. This method unfortu-
nately has its typographical limitations but it is hoped that they in no way distract
the reader.
Printed in Great Britain by William Clowes & Sons, Limited
London, Beccles and Colchester
PREFACE
This book was originally planned as a second edition of the
previous book "FIELD IONIZATION MASS SPECTROMETRY" (Pergamon
Press) which was written mainly in 1969, but which did not
appear before 1971. When the author of this book looked at the
situation about six years later, he realized that the development
of this area of research had increased so rapidly that he
decided to write an essentially new book and to take over only
some passages of the old one. The concept was to rearrange the
contents according to the five main subareas which proved to be
of growing importance:
Theory of Field Ionization and Field Desorption, Experimental
Techniques, High Field Surface Chemistry, Kinetics and Mechanisms
of Decomposition of Field Ions in the Gas Phase, Qualitative and
Quantitative Analysis with Field Ionization and Field Desorption
Mass Spectrometry. Further, it was no longer possible to refer
to virtually all publications which had appeared ( as in the
first book), but rather to select the essential studies
published by the end of 1976. Consequently, the title of the
book was changed to "PRINCIPLES OF FIELD IONIZATION AND FIELD
DESORPTION MASS SPECTROMETRY".
The book could not have been finished without the invaluable
aid of many people. First, I would like to thank Douglas ßarofsky
who very carefully translated chapter one and two from German to
English. Moreover, he made numerous critical comments and
proposals to improve the quality of those chapters. A few pages
were added after his translation which he could not check.
Therefore, I don't blame him for errors in style or contents.
Chapters three to five were written in English from the
beginning. Andrew Thorne, a former student from Oxford, improved
the English of chapter three.It was looked at from a scientific
aspect by Franz Wilhelm Röllgen and Hans Josef Heinen to whom I
am much indebted. Chapter four was read first by Peter Derrick,
who gave some critical and useful comments, and who removed the
worst errors of my English. Many new suggestions have since then
been made on chapter four by Karsten Levsen who corrected errors
and, even more important, proposed a new "red line" for making
xi
xii Preface
this chapter much clearer.
The English of chapter five was improved again by Andrew
Thorne and it was looked through scientifically by Wolf Dieter
Lehmann and to some extent by Hans-Rolf Schulten.
Last, but not least, my most sincere thanks go to Mrs. Hanne Kret-
schmer and Mrs. Dagmar Aston who typed the text on the Pergamon
Press camera-ready laysheets. They untiringly carried out all
changes in the text which seemed to be mandatory.
INTRODUCTION
Field ionizationt and field desorption mass spectrometry is a
rather new branch of mass spectrometry which originated
historically from FI microscopy as founded by Erwin Muller in
1953. The first quantitative and comprehensive investigation on
Fl mass spectrometry was published in 1954 by Inghram and
Gomer (1,2). The ion source for the mass spectrometer constructed
by these authors could be used as an FE microscope as well.
In another paper, Muller and Bahadur (3) reported on visual
observation of ions which had been separated according to their
mass by a magnetic field, on the luminescent screen of an Fl
microscope. The close relation between FI or FE microscopy and
FI mass spectrometry still remained obvious in these two papers.
The author of this book started with a systematic
investigation of the new discipline Field Ionization Mass
Spectrometry in 1957, in a manner which did not make use of
simultaneous or alternative microscopic observations. It was
entirely concentrated on FT in connection with mass spectrometric
problems in order to lead the new method to numerous applications
in the area of physico-chemical investigations and quantitative
or structural chemical analyses (4-6).
At the beginning the number of authors working in this new
field was very small. Nearly simultaneously with Inghram and
Gomer, Drechsler (7) had pointed out the principle of an Fl
source for mass spectrometric analyses which was realized
experimentally a number of years later; Schmidt (8) reported on
t FT is used throughout this book for"field ionization", FD for
"field desorption", FE for "field emission" and EI for "electron
impact".
xiii
xiv Introduction
the design, and on experiments, with an improved version of this
source. Block (9) started relatively early with applications of
the FI mass spectrometric method to problems of adsorption and
catalysis. Robertson (10) contributed on SI mass spectra taken
with sharp metal edges. The names and the work of most authors
having published on FI and FD mass spectrometry after the
authors named so far will be discussed in the course of this
book.
The main reason for the initially rather slow development
of the new field were among others:
The number of spectra available was still too small to
obtain an insight into the general properties of FI mass spectra;
the theory was not yet sufficiently worked out for a good
understanding of regularities observed in the FI spectra of
homologous series of organic substances; the instrumental
techniques were much less developed than today; there were only
some rough ideas on what the main application of the new method
would be in the future; and last but not least, there were no
commercial FI/FD sources for mass spectrometry available.
The number of users of FI mass spectrometers has been
increasing, however, very quickly during the last few years, and
a further quick extension of the area occurred after the recent
introduction of commercial FI/FD sources, especially sources
which are combined with electron impact facilities.
During the last years various groups are found to be
working in five main areas of FI/FD mass spectrometry:
1. Theory of field ionization and field desorption.
2. Instrumental development (mainly field anodes for FI and FD
ion sources, adaption of ion sources to the different types
of mass spectrometers, sample loading techniques, temperature
programming, etc.).
3. "High field chemistry", heterogeneous catalysis.
4. Kinetics and mechanisms of decomposition of organic ions in
the gas phase within the time range of 10 11 to 10-5 sec.
Introduction xv
5. Qualitative and quantitative analyses with the FI and FD
mass spectrometers.
A brief comment will now, in the remaining space of this
introduction, be given on the overall design of FI/FD mass
spectrometers. The most specific part of these instruments is
the FI/FD source which strongly differs in design and function
from EI sources. On the other hand, the mass spectrometers
connected with these FI/FD sources are of conventional design.
Principles and design of the mass spectrometers themselves will
therefore not be described in this book. There exists a number of
excellent monographs on mass spectrometry which the reader may
use as a general introduction to the principles and the design of
mass spectrometers.
All mass spectrometers could be used, in principle, in
connection with FI/FD sources which are used in combination with
EI sources, i.e. magnetic mass spectrometers or time-of-flight,
high frequency, quadrupole, monopole or other mass spectrometers.
1. THEORY OF FIELD IONIZATION (FI)
AND FIELD DESORPTION (FD)
1.1 Fl PROBABILITY
Atoms or molecules are field ionized when they are
subjected to high electric fields of the order of 107 to several
times 108 V/cm. The first experimental evidence for FI was Rausch
von Traubenberg's (11) observation that certain Stark effect
lines in the optical spectrum of highly excited hydrogen atoms
disappear if the gas is acted upon by an electric field of
several times 1O5 V/cm. It was concluded from this experimental
result that the lifetimes of highly excited electronic states are
strongly reduced in the presence of high electric fields. Field
strengths of the order of 108 V/cm are required for Fl to take
place from the ground state of an atom or molecule instead of
from a highly excited state.
The mechanism of FI can be explained theoretically on the
basis of the wave properties of matter. The probability for FI to
occur from the ground state of a hydrogen atom was first
calculated by Oppenheimer (12). More general, theoretical
treatments have since been given by Kirchner (13), Inghram and
Gomer (2), and Müller and Bahadur (3).
The theory of FI developed by these authors (2,3,13) may
be discussed with the aid of a potential energy diagram. The
potential energy of the valence electron of an atom is drawn in
Fig. 1.1 as a function of distance from an ideally smooth metal
surface. The metal extends through the region r < O. The region
r > O is vacuum. The surface of the metal is assumed to be
perfectly clean, i.e. without adsorbed layers. The conduction
band of the metal is filled with electrons up to the Fermi
level ii . An amount of energy equal to the work function must
4
be transferred to an electron at the Fermi level in order to
remove it from the metal to infinity.
1
2 Theory of FI and FD
Given an atom, such as a hydrogen atom, at a distance d
from the metal surface, the electronic ground state of the
valence electron would, in general, be as indicated in Fig. 1.1.
The ionization energy I of the atom must be supplied to the
electron in order to remove it to an infinitely large, positive
value of r. The electron energy diagram for a free atom is drawn
in Fig. 1.la with a solid line.
n
5f
m /~~///////// t
T -O
Electronic
Metal ground
level
a) b)
Fig. 1.1. Potential energy V(r) of the valence
electron of an atom in the vicinity of a metal
surface: (a) without field, (b) with field.
I = ionization energy; f = work function of the
metal; V. = image potential (dashed line);
VF = eFr2= potential energy due to external
electric field (F = field strength); m = Fermi
level.
As will be described below, an image potential V must be
i
taken into account at small distances from the metal surface. In
the case of FI, Fig. 1.1b, the potential energy due to the
electric field eFr must be superimposed on that of the atom. The
resulting potential energy for an electron, located a distance r
from the surface along a line perpendicular to the surface and
passing through the ion, was approximated by Muller and Bahadur
(3) for a hydrogen atom by the expression
_ 2 2 2
e
V(r) = 4r + +r
k
