Photoelectron Spectroscopy. An Introduction to Ultraviolet Photoelectron Spectroscopy in the Gas Phase

275 Pages·1984·3.216 MB·English

by  J. H. D. Eland (Auth.)

Photoelectron Spectroscopy
AN INTRODUCTION TO ULTRAVIOLET PHOTOELECTRON
SPECTROSCOPY IN THE GAS PHASE
Second Edition
J.H.D. ELAND, MA, DPhii
Fellow of Worcester College, Oxford
Butterworths
London Boston Durban Singapore Sydney
Toronto Wellington
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First published 1984
Butterworth & Co (Publishers) Ltd, 1984
British Library Cataloguing in Publication Data
ElandJ.H.D.
Photoelectron spectroscopy.—2nd ed.
1. Photoelectron spectroscopy
I. Title
535.8'44 OC454.P48
ISBN 0-408-71057-8
Library of Congress Cataloging in Publication Data
Eland, J. H. D.
Photoelectron spectroscopy.
Includes index.
1. Photoelectron spectroscopy. I. Title.
OC454.P48E38 1983 543'.0858 83-14386
ISBN 0-408-71057-8
Typeset by Scribe Design, Gillingham, Kent
Printed and bound in Great Britain by the Camelot Press Ltd.,
Southampton
Preface to the second edition
The expansion of photoelectron spectroscopy foreseen in the
preface to the first edition has indeed taken place in the ten years
since 1972. Besides the ever expanding applications of the techni-
que in different branches of chemistry, there have been consider-
able advances in our understanding of the photoionization process
and of the ion reactions that follow it. Progress has been stimu-
lated by improvements in experimental technique, particularly by
the emergence of lasers and electron storage rings as new sources
of ionizing radiation. At the same time theoretical chemists have
learnt how to calculate ionization energies in a much more
sophisticated and reliable way, by going beyond Koopmans'
theorem and the orbital model. These developments have promp-
ted the preparation of a second edition.
The purpose of the book remains unchanged, being to equip
advanced students and researchers starting in photoelectron spec-
troscopy, or turning to photoelectron spectroscopy from their own
specialities, with a complete account of the subject at a practical
level. All parts of the book have been revised and major changes
have been made in Chapters 2 (Experimental Methods), Chapter 3
(Ionization) and Chapter 7 (Reactions of Positive Ions). New
sections on such topics as multi-photon ionization, synchrotron
radiation and rotational effects have inevitably added to the length
of the text, but some other topics, which have proved after the
passage of time to be of less pressing interest, have been omitted.
The references throughout the text are again intended to be useful
rather than archival, and to any authors who might feel aggrieved
by lack of reference to their vital work, I can only point out that
they are undoubtedly cited indirectly. If there is a bias it is towards
those authors who have sent me their reprints and to whom I
express my thanks most heartily for thereby lightening the task of
preparation.
John H.D. Eland
Preface to the first edition
The photoelectron spectrometer will soon take its place in the
laboratory beside the mass spectrometers, optical spectrometers
and radio-frequency spectrometers that have become routine tools
of the chemist and physicist. A new form of molecular spectro-
scopy naturally requires an incubation period in the hands of
specialist physicists and physical chemists before it becomes useful
in wider fields of chemistry, and photoelectron spectroscopy is
now emerging from such a stage in its development. Sure signs of
this emergence are the burgeoning of chemical applications of the
technique and the availability of commercial photoelectron spec-
trometers with very high performance. At the same time, there is a
lack of any textbook that covers the new technique at an advanced
undergraduate or first year research level, and this I have attemp-
ted to provide. My aim has been to cover, at least qualitatively,
almost all that a chemist needs to know in order to interpret a
photoelectron spectrum with which he is confronted. The treat-
ment is experimentally based and non-mathematical, but assumes
some familiarity with other spectroscopic techniques and with the
chemical applications of Group Theory.
The importance of photoelectron spectroscopy in the study of
molecular electronic structure is now widely appreciated; its
relevance to mass spectrometry and unimolecular reaction rate
theory deserves more attention than it has hitherto received, and I
hope that the inclusion of Chapter 7 on ionic dissociation will go
some way to rectify this. Chapters 1 to 6 form a progressive
introduction to photoelectron spectroscopy, and they are intended
to be read sequentially, with a few possible exceptions. The more
difficult topics in Sections 1.4.2, 3.4.3, 3.5 and 4.6 could be
omitted on a first reading and Chapter 2, on experimental
methods, may be referred to separately from the main text. The
final chapter contains accounts of some selected applications of
photoelectron spectroscopy in chemistry, and includes a sufficient-
ly full reference list for these topics to be followed up in detail.
Shorter reference lists are provided for all the other chapters and
should serve as a key to the literature, but they are by no means a
complete bibliography; often only the most recent papers on a
particular subject are cited. In a rapidly advancing field such as
this, it is impossible to write a completely up-to-date book, and the
inclusion of new material had to stop at the end of 1972.
In preparing this book I have been helped by discussions with
several scientists, and I should like to thank Dr. B. Brehm, Dr.
M.S. Child, Dr. C.J. Danby, Professor E. Heilbronner and Mr.
A.F. Orchard in particular. Dr. Brehm and Dr. Danby also read
parts of the manuscript in draft and made suggestions for several
necessary improvements. The typing was undertaken by Mrs. M.
Long, and I am most grateful for her speed and cheerfulness in
dealing with a difficult manuscript. Finally, I want to thank my
wife, leva, for the immense amount of help she has given at every
stage of the work.
John H.D. Eland
1
Principles of photoelectron
spectroscopy
1.1 Introduction
When light of short wavelength interacts with free molecules, it
can cause electrons to be ejected from the occupied molecular
orbitals. Photoelectron spectroscopy is the study of these photo-
electrons, whose energies, abundances and angular distributions
are all characteristic of the individual molecular orbitals from
which they originate. The experimental singling-out of individual
molecular orbitals is the outstanding feature of photoelectron
spectroscopy, and one which distinguishes it from all other
methods of examining molecular electronic structure.
The quantity measured most directly in photoelectron spectro-
scopy is the ionization potential for the removal of electrons from
different molecular orbitals. According to an approximation
known as Koopmans' theorem, each ionization potential, I is
p
equal in magnitude to an orbital energy, 6j!
/ j= - 8 j (1.1)
This is an approximation additional to those inherent in the
molecular orbital model for many-electron systems, but it is a good
and a very useful one. It means that the photoelectron spectrum of
a molecule is a direct representation of the molecular orbital
energy diagram. Furthermore, the spectrum also shows, from the
detailed form of the bands, what changes in molecular geometry
are caused by removal of one electron from each orbital. These
changes reveal the character of the orbitals, whether they are
bonding, antibonding or non-bonding, and how their bonding
power is localized in the molecules.
1
2 PRINCIPLES OF PHOTOELECTRON SPECTROSCOPY
The photoelectron spectroscopy discussed in this book is based
on photoionization brought about by radiation in the far ultra-
violet region of the spectrum. It was.invented ea1r2l,y in the 1960s
independently by t3wo groups, one led by Turn4er in London, the
other by Vilesov in Leningrad. Siegbahn and his group at
Uppsala had evolved a similar technique a little earlier, but they
used X-radiation instead of ultraviolet light and at first concen-
trated more on the study of solids than on free molecules. Both
ultraviolet and X-ray photoelectron spectroscopy have been exten-
sively developed and have found many applications in chemistry
and physics. This book is an attempt to present an up-to-date view
of ultraviolet photoelectron spectroscopy, and reference to X-ray
work is made only when it expands upon or illuminates aspects of
the valence electronic structure of molecules or ions.
1.2 Main features of photoelectron spectra
In a photoelectron spectrometer, an intense beam of mono-
chromatic (monoenergetic) ultraviolet light ionizes molecules or
atoms of a gas in an ionization chamber:
M + Av^M.001 + e (1.2)
The light used is most commonly the helium resonance line He I at
584 A (58.4 nm), which is equivalent to 21.22 electronvolts (eV) of
energy per photon. This energy is sufficient to ionize electrons
from the valence shell of molecules or atoms, that is, from the
orbitals that are involved in chemical bonding and are character-
ized by the highest principal quantum number of the occupied
atomic orbitals. In each orbital, j, of an atom or molecule, the
electrons have a characteristic binding energy, the minimum
energy needed to eject them to infinity. Part of the energy of a
photon is used to overcome this binding energy, I and if the
v
species is an atom the remainder, hv — l must appear as kinetic
v
energy (KE) of the ejected electrons:
KE = hv-I (1.3)
]
The ejected photoelectrons are separated according to their
kinetic energies in an electron energy analyser, detected and
recorded. The photoelectron spectrum is a record of the number
of electrons detected at each energy, and contains a peak at each
energy, hv — l corresponding to the binding energy, I of an
v v
MAIN FEATURES OF PHOTOELECTRON SPECTRA 3
Electron Photoelectron
energy spectrum
hv-I2
Free
electrons
hv-L
hv
0 h Electron signal
Bound
electrons
•a i -H-
-K-
Figure 1.1 Idealized photoionization process and photoelectron
spectrum of an atom
electron in the atom, as illustrated schematically in Figure 1.1. If
the species is a molecule, there are the additional possibilities of
vibrational or rotational excitation on ionization, so the energies
of the photoelectrons may be reduced:
KE =/*v - /; - £* v , irot b (1.4)
The spectrum may now contain many vibrational lines for each
type of electron ionized, and the system of lines that corresponds
to ionization from a single molecular orbital constitutes a band.
Apart from Koopmans' theorem, there are two approximate
rules that make the relationship between photoelectron spectra
and molecular electronic structure especially simple:
(1) Each band in the spectrum corresponds to ionization from a
single molecular orbital.
(2) Each occupied molecular orbital of binding energy less than
hv gives rise to a single band in the spectrum.
4 PRINCIPLES OF PHOTOELECTRON SPECTROSCOPY
Because of these rules, the photoelectron spectrum is a simple
reflection of the molecular orbital diagram, as illustrated in Figure
1.1. The rules are a simplification, however, and there are three
reasons why there may, in fact, be more bands in a spectrum than
there are valence orbitals in a molecule. Firstly, additional bands
are sometimes found that correspond to the ionization of one
electron with simultaneous excitation of a second electron to an
unoccupied excited orbital. This is a two-electron process, and in
the ionization potential region below 20 eV the bands produced in
the spectrum are much weaker than simple ionization bands.
Secondly, ionization from a degenerate occupied molecular orbital
can give rise to as many bands in the spectrum as there are orbital
components, because although the orbitals are degenerate in the
molecule they may not be so in the positive ion. The mechanisms
that remove the degeneracy are spin-orbit coupling and the
Jahn-Teller effect. Thirdly, ionization from molecules such as 0
2
or NO, which have unpaired electrons, can give many more bands
than there are occupied orbitals in the molecule, and in such
instances neither Koopmans' theorem nor the simple rules apply.
In order to introduce these main features of photoelectron
spectra, it is convenient to take practical examples, starting with
the spectra of atoms and proceeding to those of more complicated
molecules. The spectroscopic names of atomic and molecular
electronic states are constantly needed when describing the spec-
tra, and any readers who are not familiar with them may find it
helpful to consult Appendix I.
1.2.1 Atoms
The photoelectron spectrum of atomic mercury excited by helium
resonance radiation is shown in Figure 1.2. The vertical scale in
this and all other photoelectron spectra is the strength of the
electron signal, usually given in electrons per second. The absolute
intensities have no physical significance because they depend on
physical and experimental factors, which, although constant
throughout the measurement of the spectrum, are not precisely
known. The relative intensities of different peaks in the spectrum
are meaningful, however, as they are equal to the relative proba-
bilities of photoionization to different states of the positive ion,
which are called the relative partial ionization cross-sections.
Three horizontal scales are given in Figure 1.2 to illustrate the
relationships between measured electron energy, ionization poten-
tial and the internal excitation energy of the ions, including
MAIN FEATURES OF PHOTOELECTRON SPECTRA 5
300h
1
~
s
s
nt
u
o
tc
nt
e
urr
c
n
o
ctr
e
el
o
ot
h
P
1 7 8 9 10 11 12 13 Electron energy
-i 1 1 1 i '
20 19 18 17 16 15 H 13 12 11 10 9 8 Ionfzation energy
_J _L
1 0 9 8 7 6 54 32 1 0 Ion excitation energy
eV
Figure 1.2 Photoelectron spectrum of mercury excited by He I (584 A)
radiation
electronic excitation energy. Although volts (V) are the units of
potential and electronvolts (eV) are units of energy, it is a usage
hallowed by tradition to speak of ionization potentials as energies
and to quote them in units of electronvolts. Nevertheless, when
energy quantities are being compared, the phrase ionization
energy' is used frequently in this book, and henceforth the only
horizontal scale given for photoelectron spectra will be one of
ionization energy in electronvolts. The SI units of energy, joules,
are very inconvenient in this field and are not used by spectroscop-
ists; conversion factors for the important units are given in
Appendix II.
The spectrum of mercury in Figure 1.2 was obtained with a
spectrometer in which electrons of only one energy at a time were
able to reach the detector; it is called a differential spectrum.
Spectra are sometimes encountered in integral form, taken with
spectrometers in which all electrons of more than a certain energy
can reach the detector simultaneously. The spectrum of mercury
measured in such a spectrometer is shown in Figure 1.3, where its
integral relationship to the+ spectrum in Figure 1.2 is apparent.
Both spectra show that Hg ions are formed by photoionization in
three electronic states, with ionization energies of 10.44,14.84 and
16.71 eV. The states involved are well known from the atomic