Table Of Content15
Advanced Serles In Physlcal Chernlstry
CONICAL
INTERSECTIONS
Electronic Structure,
Dynamics & Spectroscopy
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15
Advanced Series in Physical Chemistry
CONICAL
INTERSECTIONS
EI e ct ron i c St ruc t u re,
Dynamics & Spectroscopy
Editors
Wolfgang Domcke
Technicul University of Munich/ Germuny
David R Yarkony
Johns Hopkins UniversiM USA
Horst Koppel
University of Heidelber~G~e rmuny
\b
World Scientific
NEW JERSEY LONDON SINGAPORE BElJlNG SHANGHAI HONG KONG TAIPEI CHENNAI
Published by
World Scientific Publishing Co. Pte. Ltd.
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USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601
UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE
British Library Cataloguing-in-Publication Data
A catalogue record for this book is available from the British Library.
CONICAL INTERSECTIONS: ELECTRONIC STRUCTURE,
DYNAMICS & SPECTROSCOPY
Copyright © 2004 by World Scientific Publishing Co. Pte. Ltd.
All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means,
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ADVANCED SERIES IN PHYSICAL CHEMISTRY
INTRODUCTION
Manyofuswhoareinvolvedinteachingaspecial-topicgraduatecoursemay
have the experience that it is difficult to find suitable references, especially
referencematerialsputtogetherinasuitabletextformat.Presently,several
excellent book series exist and they have served the scientific community
well in reviewing new developments in physical chemistry and chemical
physics. However, these existing series publish mostly monographs consist-
ing of review chapters of unrelated subjects. The modern development of
theoretical and experimental research has become highly specialized. Even
inasmallsubfield,experimentalortheoretical,fewreviewersarecapableof
giving an in-depth review with good balance in various new developments.
A thorough and more useful review should consist of chapters written by
specialists covering all aspects of the field. This book series is established
with these needs in mind. That is, the goal of this series is to publish
selected graduate texts and stand-alone review monographs with specific
themes, focusing on modern topics and new developments in experimen-
tal and theoretical physical chemistry. In review chapters, the authors are
encouraged to provide a section on future developments and needs. We
hope that the texts and review monographs of this series will be more use-
ful to new researchers about to enter the field. In order to serve a wider
graduate student body, the publisher is committed to making available the
monographs of the series in a paperbound version as well as the normal
hardcover copy.
Cheuk-Yiu Ng
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PREFACE
TheBorn–Oppenheimeradiabaticapproximationrepresentsoneofthecor-
nerstones of molecular physics and chemistry. The concept of adiabatic
potential-energy surfaces, defined by the Born–Oppenheimer approxima-
tion, is fundamental to our thinking about molecular spectroscopy and
chemical reaction dynamics. Many chemical processes can be rational-
ized in terms of the dynamics of the atomic nuclei on a single Born–
Oppenheimer potential-energy surface. Nonadiabatic processes, that is,
chemical processes which involve nuclear dynamics on at least two coupled
potential-energysurfacesandthuscannotberationalizedwithintheBorn–
Oppenheimer approximation, are nevertheless ubiquitous in chemistry,
most notably in photochemistry and photobiology. Typical phenomena
associated with a violation of the Born–Oppenheimer approximation are
the radiationless relaxation of excited electronic states, photoinduced uni-
molecular decay and isomerization processes of polyatomic molecules.
During the last few decades, we have witnessed a change of paradigms
in nonadiabatic chemistry. First, the remarkable advances achieved in fem-
tosecond laser technology and time-resolved spectroscopy have revealed
that the radiationless decay of excited electronic states may take place
much faster than previously thought. The traditional theory of radia-
tionless decay processes, developed in the sixties and seventies of the
last century, cannot explain electronic decay occurring on a time scale
of few tens of femtoseconds. Second, the development and widespread
application of multi-reference electronic structure methods for the cal-
culation of excited-state potential-energy surfaces have shown that so-
called CONICAL INTERSECTIONS of these multidimensional surfaces,
predicted by von Neumann and Wigner in 1929, are the rule rather than
vii
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viii Preface
the exception in polyatomic molecules. As a result, the concept of coni-
cal intersections has become widely known in recent years. That conical
intersections may be responsible for ultrafast radiationless processes has
been surmised as early as 1937 by Teller. Nowadays, it is increasingly rec-
ognized that conical intersections play a key mechanistic role in molecular
spectroscopy and chemical reaction dynamics.
There are a number of intricate issues associated with conical inter-
sections, concerning both the electronic structure, the nuclear dynamics,
and the interaction of the molecule with the radiation field. Few of us are
familiar with all these aspects of the problem, which have been analyzed
in research papers scattered throughout the chemical physics literature. It
is the intention of the present book to make this knowledge available to
interested graduate students and researchers in the thriving field of femto-
chemistry.
The chapters of this book are all theoretical in character. This reflects
the fact that the conical intersection is a theoretical concept, and as such
is not directly accessible to experimental observation. Nevertheless, the
concepts, techniques and results discussed in this book are crucial for the
interpretation of the observations in time-resolved spectroscopy and chem-
ical kinetics on femtosecond time scales. It is hoped, therefore, that this
book is of value not only for the theoretician, but also for the practitioneer
in molecular spectroscopy, photochemistry, and collision-induced reaction
dynamics.
Wolfgang Domcke, Munich
David R. Yarkony, Baltimore
Horst K¨oppel, Heidelberg
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HISTORICAL INTRODUCTION
Josef Michl
Department of Chemistry and Biochemistry,
University of Colorado,
Boulder, CO 80309-0215
In the last half century, our understanding of the qualitative features of
organicphotochemistryhasundergoneaspectaculartransformation.Inthe
mid-1900s,thecommunityofchemistsinvestigatinglight-inducedreactions
ofpolyatomicorganicmoleculeswasnotevensurethatthetheoreticalcon-
ceptsalreadywellestablishedinthegroundelectronicstate,suchasasingle
potential energy surface dictating nuclear motions, would be applicable to
electronically excited molecules. How useful would the Born–Oppenheimer
approximation be, given the energetic proximity of other electronic states?
Wouldtherebetimeforvibrationalequilibrationandcouldtransitionstate
theory be applied? Kasha’s and Vavilov’s early formulation of their impor-
tant empirical rule provided a ray of hope that at least the lowest excited
state of each multiplicity could be thought of in such familiar terms. Of
course, although the existence of metastable excited states was known,
their recognition as triplet states and the appreciation of their importance
in organic photochemistry were yet to come.
It was clear that in large organic molecules internal conversion from
upper to lower excited states occurs on a subpicosecond scale. In the many
organic molecules that exhibit no emission, such ultrafast conversion evi-
dently occurred all the way to the ground electronic state, in some cases
accompanied by a chemical transformation, in others not. When a chemi-
cal transformation did occur, it seemed to be almost always accompanied
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