Table Of ContentTopics in
vol 2
Physical Chemistry
Edited by H. Baumgartel, E. U. Franck, W. Griinbein
On behalf of Deutsche Bunsen-Gesellschaft flir Physikalische Chemie
Topics in
Physical Chemistry
vol 1 Introduction to
Surface Physical Chemistry
K. Christmann
vol 2 Gaseous Molecular Ions
An Introduction to Elementary Processes
Induced by Ionization
E. Illenberger, J. Momigny
E. Illenberger, J. Momigny
Gaseous Molecular Ions
An Introduction to Elementary
Processes Induced by Ionization
; Springer-Verlag Berlin Heidelberg GmbH •
Authors' addresses: Prof. Dr. Jacques Momigny
Prof. Dr. Eugen Illenberger Institut de Chimie
Institut fUr Physikalische Departement de Chimie
und Theoretische Chemie Generale et de Chimie-Physique
der Freien UniversiHit Berlin Universite de Liege
TakustraBe 3 Sart Tilman
D-lOOO Berlin 33 B-4000 Liege 1
Edited by:
Deutsche Bunsen-Gesellschaft
fUr Physikalische Chemie e. V.
General Secretary Dr. Heinz Behret
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Die Deutsche Bibliothek - CIP-Einheitsaufnahme
Gaseous molecular ions: an introduction to elementary
processes induced by ionization I E. Illenberger; J. Momigny.
(Topics in physical chemistry; Vol. 2)
ISBN 978-3-662-07385-8 ISBN 978-3-662-07383-4 (eBook)
DOI 10.1007/978-3-662-07383-4
NE: Illenberger, Eugen; Momigny, Jacques, GT
ISSN 0941-2646
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Copyright © 1992 by Springer-Verlag Berlin Heidelberg
Originally published by Dr. Dietrich SteinkopffVerlag GmbH & Co. KG, Darmstadt in 1992
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Preface
Most of the matter in our solar system, and, probably, within the whole universe,
exists in the form of ionized particles. On the other hand, in our natural environ
ment, gaseous matter generally consists of neutral atoms and molecules. Only under
certain conditions, such as within the path oflightning or in several technical devices
(e. g. gas discharges, rocket engines, etc.) will some of the atoms and molecules be
ionized. It is also believed that the chemistry of the earth's troposphere predomi
nantly proceeds via reactions between neutral particles. (The complex system of
atmospheric chemistry will be treated in one of the forthcoming volumes to this
series.) Why, then, are ions considered so important that hundreds oflaboratories all
over the world (including some of the most prestigious) are involved in research pro
grams on ions, covering many different facets, from biochemistry to physics? One
may obtain as many different answers as there are research groups busy in this field.
There is, however, one simple, common feature which makes it attractive to work
with ions: since they carry one or more net elementary charges, they can easily be gui
ded, focused or separated by appropriate electric and magnetic fields, and, last but
not least, they can easily be detected.
Apart from these advantages, which are welcome and appreciated by the
researcher, the study of molecular ions can provide insight into very fundamental
aspects of the general behavior of molecules. Moreover, the ionization process itself
can be used to obtain information on certain properties or quantities of the corre
sponding neutral molecule. Mass spectrometry, for example, is a widely used and
established technique to analyze neutral particles. This is usually performed by elec
tron-impact ionization of the neutral gas sample, and separation of the generated
ions (in space or time) according to their molecular weight.
In photoelectron spectroscopy, on the other hand, the kinetic energy of the ion
ized electrons is recorded, and the spectrum obtained is an image of the energy lev
els of the molecular orbitals in the neutral molecule. Photoelectron spectroscopy
had (and still has) an enormous impact on theory, particularly on quantum chemis
try in the development of sophisticated molecular orbital calculations. More refined
techniques in photoionization such as coincidence techniques between electrons
and ions allow a detailed study of unimolecular reactions in that one can follow the
evolution of a molecular ion prepared in a definite state (i. e., with a defined amount
of internal energy). General concepts to describe unimolecular processes were first
developed some decades ago, relating to fragmentation patterns observed in mass
spectra. The unimolecular decomposition of energized ions is a key subject of this
volume. Today, unimolecular processes in neutrals can be studied in great detail by
laser techniques (pump and probe experiments, i. e., pumping a defined amount of
energy into a molecule with a first laser, and probing the products (identity and state)
by means of a second one). Such processes will be treated in detail in one ofthe forth-
VI Preface
coming volumes of this series (Modem Photochemistry). Due to the limited wave
length range of available laser systems, however, these methods are not as generally
applicable as photoionization.
Negatively charged ions playa particular role, since they can be formed in the gas
phase in large quantities by attachment of very low-energy electrons (sometimes
near 0 eV ). In contrast, the formation of positive ions requires an energy equal to or
greater than the first ionization energy, which is around 10 eV for most organic mole
cules. Low-energy electrons can be very reactive, in that they are effectively captured
by many molecules, which then undergo rapid unimolecular decompositions. The
cross-section for such processes can be very large, i. e., 4 to 5 orders of magnitude lar
ger than typical photoionization or excitation cross-sections. If a slow electron col
lides with an excited molecule, dissociative capture processes can have enormous
cross-sections, many orders of magnitude larger than the geometrical cross-section
of the respective molecule. The role of slow electrons and negative ions has some
how been disregarded, and comparatively few groups are working in that area (at
least in Germany). Part II of this volume is particularly dedicated to negative ions
and their reactions in low-energy electron collisions.
Removing or adding electrons from or to molecules, along with the reaction ofthe
ionized species, may be regarded as a particular step in relation to a chemical reac
tion. Of course, a chemical reaction occurring in solution is a much more complex
process as it may proceed via energy and charge transfer between different molecules
(the educts), thereby cleaving bonds and forming new molecules (the products).
Approaches toward a microscopic study of chemical reactions are currently per
formed in molecular beam experiments, particularly in supersonic beams contain
ing molecular aggregates (or clusters). Such weakly bound aggregates represent a
link between gaseous and condensed matter. Some examples in the case of reactions
following electron attachment to van der Waals clusters are discussed in the last sec
tions of this book.
This volume contains three parts and is organized as follows: the first part gives a
general overview of the experimental methods used to prepare positive and negative
ions, and how their evolution can be studied. In the case of positive ions, it addresses
the "classical" techniques of photoionization, i. e., ionization by VUV light sources.
Although mentioned occasionally, ionization bymultiphoton laser techniques is not
treated explicitly. The basic instrumentation (light sources, mass spectrometers,
electron energy filters, and detectors) is introduced, with the emphasis placed on the
description of the operation principles rather than on sophisticated technical details.
Part II (J. Momigny) focuses on general processes which occur when a molecule is
subjected to the absorption of energy above its first ionization limit, i. e., direct ioni
zation and autoionization, the energy flow via radiative and nonradiative transitions
between electronic states, including nonadiabatic interactions, and the decomposi
tion into fragments. Besides photoionization, other techniques for preparing posi
tive ions (charge exchange, Penning ionization, field ionization, etc.) are described.
Statistical approaches to calculate the rate constant for a unimolecular reaction and
the excess energy distribution among the fragments are introduced. The obtained
Preface VII
results are always compared with experimental values in order to test the validity and
limits of the approaches.
The last part is devoted to negative ions, their behavior and peculiarities in com
parison to positive ions. Electron-capture processes for some prototype systems are
presented and discussed. Particular emphasis is placed on the question of how the
relevant quantities, e. g., attachment energies, selection rules for populating nega
tive ion states, reaction products and their energy distribution behave on proceeding
from an isolated molecule to clusters of increasing size.
Most of the work presented in this last part has generoulsy been supported by the
Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie which is
gratefully acknowledged. Thanks are also due to many colleagues and coworkers for
valuable contributions. Their names appear in the original publications cited here.
It is hoped that this text will help to bridge the gap between the body of general
knowledge and specific current research, and thus being of benefit in initiating the
student and newcomer in the subject of gaseous molecular ions.
I would like to thank the publisher Dr. Dietrich SteinkopffVerlag, Darmstadt, par
ticularly the Chemistry Editor Dr. Maria Magdalene Nabbe for her constructive col
laboration and the editors for their invitation to contribute to this series.
Special thanks are due to Mrs. L. Brodricks for carefully processing the original
manuscript.
Eugen Illenberger Berlin, December 1991
Contents
Preface. . .. . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . .. . . . . . . .. .. V
Part I Preparation and Decomposition of Positive and Negative Ions:
Experimental Techniques and Instrumentation
E. Illenberger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1 Some General Remarks on Positive and Negative Ions,
Photoionization, and Electron Attachment ..................... . 1
References .................................................. . 6
2 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1 Photoelectron Spectroscopy (PES) . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Photo ionization Mass Spectrometry (PIMS) .................. 11
2.3 Photoelectron-Photoion Coincidence Spectroscopy (PEPICO). . . .. 13
2.4 Electron Attachment Spectroscopy (EAS). . . . . . . . . . . . . . . . . . . .. 17
2.5 Electron Transmission Spectroscopy (ETS). . . . . . . . . . . . . . . . . . .. 21
2.6 Photo detachment Spectroscopy ............................ 25
References ................................................... 27
3 Instrumentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 30
3.1 Light Sources and Monochromators. . . . . . . . . . . . . . . . . . . . . . . .. 30
3.1.1 Line Sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 30
3.1.2 Continuum Sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 31
3.1.3 Dispersive Elements / Monochromators . . . . . . . . . . . . . . . . . . . . .. 33
3.2 Electron Energy Analyzers and Monochromators. . . . . . . . . . . . . .. 36
3.2.1 Retarding Field Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 37
3.2.2 Parallel Plate Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 39
3.2.3 1270 Cylinder Filter ..................................... 42
3.2.4 Hemispherical Analyzer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 43
3.2.5 Cylindric Mirror Analyzer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 44
3.2.6 Trochoidal Electron Filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 45
3.2.7 Wien Filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 46
3.2.8 Threshold Photoelectron Analyzer. . . . . . . . . . . . . . . . . . . . . . . . .. 47
Contents IX
3.2.9 Further Spectrometers and Limiting Effects on Energy
Resolution ............................................ 48
3.3 Mass Spectrometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 51
3.3.1 Principles of Mass Analysis ............................... 51
3.3.2 Double Focusing Magnetic Sector Mass Analyzers. . . . . . . . . . . . .. 54
3.3.3 Time-of-Flight Mass Spectrometers and Reflectrons. . . . . . . . . . . .. 57
3.3.4 Quadrupole Mass Spectrometers ........................... 64
3.3.5 Ion Cyclotron Resonance (lCR) Mass Spectrometers. . . . . . . . . . .. 67
3.3.6 Other Mass Analyzers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 69
3.4 Detectors ............................................. 70
3.4.1 Faraday Cup Collectors .................................. 70
3.4.2 Electron Multipliers, Channeltrons, Channelplates . . . . . . . . . . . . .. 71
3.4.3 VUV Light Detectors .................................... 73
References ................................................... 74
Part II The Monomolecular Decay of Electronically Excited Molecular Ions
J. Momigny . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 81
Introduction ................................................. 81
1 Ionization Processes in Gaseous Phase . . . . . . . . . . . . . . . . . . . . . . .. 83
1.1 Resonant Photon Absorption and Photoionization . . . . . . . . . . . . .. 83
1.1.1 Absorption and Photo ionization Cross-Sections; Ion Yields;
Mass Spectrum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 83
1.1.2 A Model for the Appearance of the Mass Spectrum of Diatomic
Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 85
1.1.3 Threshold Laws and Ionization Yield Curves for Diatomic Molecules 87
1.1.4 Time Windows for the Appearance of the Mass Spectrum. . . . . . .. 91
1.1.5 Origin and Decay of Superexcited States (SE) . . . . . . . . . . . . . . . .. 93
1.1.5.1 Origin of SE in Atoms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 93
1.1.5.2 Origin of Superexcited States in Moleclues ................... 94
1.1.5.3 Non Franck-Condon Populations of Electronic States oflons
through the Decay of Superexcited States .................... 95
1.1.6 Ionization Yield Curves for Polyatomic Molecules. . . . . . . . . . . . .. 97
1.1.7 Concluding Remarks .................................... 98
1.2 Refined Details about Photo ionization Processes.
Photoelectron Spectroscopies. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 98
1.2.1 Photoelectrons as Footprints of the Ionization Processes. . . . . . . .. 98
1.2.2 Non-Resonant Photoelectron Spectroscopy. . . . . . . . . . . . . . . . . .. 99
x Contents
1.2.3 Photoelectron Spectrum and Molecular Orbitals 100
1.2.4 Constant Photoelectron Energy Spectroscopy. . . . . . . . . . . . . . . . .. 105
1.3 Photo ion-Photoelectron Coincidence Methods ................ 107
1.3.1 Generalities about Photo ion-Photoelectron Coincidence Methods . 107
1.3.2 Non-Resonant PIPECO Mass Spectra ....................... 108
1.3.3 Resonant PIPECO Mass Spectra ........................... 112
1.3.4 From Photoionization Yield Curves to the Breakdown Diagram:
an Old-Fashioned Way . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 112
1.4 Electroionization ....................................... 114
1.4.1 Resonant Electroionization ................................ 114
1.4.2 High-Energy Electron Impact: Photon Simulation .............. 115
1.5 Charge Exchange Ionization ............................... 119
1.5.1 Introduction to Charge Exchange Physics and to Charge
Exchange Mass Spectra .................................. 119
1.5.2 A first Approach to the Measurement of the Rate Constant for
Dissociative Ionization as a Function of Internal Energy. . . . . . . .. 122
1.6 Field Ionization ........................................ 124
1.6.1 Description of the Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 124
1.6.2 Lifetimes of Field Ionized Molecular Ions .. , . . . . . . . . . . . . . . . .. 125
1.7 Penning Ionization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 126
1.7.1 General Description and Considerations ..................... 126
References ............... '. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 128
2 The Occurrence of Transitions between the Electronic States of
Molecular Ions ......................................... 132
Introduction ................................................. 132
2.1 Non-Radiative Transitions between Electronic States ............ 132
2.1.1 Non-Adiabatic Interactions between Electronic States ........... 132
2.1.2 Allowed and Avoided Crossings between Potential Energy Curves . 133
2.1.3 Crossings between Energy Hypersurfaces - Conical Intersections .. 135
2.14 Non-Adiabatic Interactions and the Time Scale for Energy
Redistribution .......................................... 137
2.2 Radiative Transitions between Electronic States of Molecular Ions . 138
References ................................................... 139
3 Energy Balance in the Dissociation Processes of Molecular Ions . . . .. 141
3.1 Experimental Approach to the Thermochemistry of Dissociation
Processes ............................................. 141
