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Electronic and Photoelectron Spectroscopy. Fundamentals and Case Studies

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Angewandte
Books
Chemie
Fundamentals and
Case Studies. By
Andrew Ellis, Miklos
Feher and Timothy
Wright. Cambridge
University Press,
Cambridge 2005.
286 pp., hardcover
£ 30.00.—ISBN
0-521-81737
This book aims to give the reader an
introduction to the analysis and interpretation of vibration- and rotationresolved electronic and photoelectron
spectra. In contrast to other books of
this kind, in which the subject is usually
built up systematically, with gradually
increasing complexity as one works
through the text, here the authors have
chosen an approach based on case
studies taken from the literature of the
past 10–15 years.
In accordance with that concept, the
book is arranged in three parts. Part 1
(64 pages) provides an introduction to
the fundamentals of optical spectroscopy, Part 2 (46 pages) describes experimental
techniques,
and
Part 3
(129 pages) contains the actual case
studies. Eight appendices (51 pages altogether) complete the work.
The essential core of the book is
certainly the third part containing the 16
case studies. The investigations on which
these are based were all carried out in
the gas phase, and mainly involve
experiments on nozzle jets. The first
example is the photoelectron spectrum
of CO, where the discussion is centered
on the connection between the photoelectron spectrum and the molecular
Angew. Chem. Int. Ed. 2005, 44, 6983 – 6984
!
Electronic and Photoelectron
Spectroscopy
orbital picture, and also on the band
contour and the change of geometry
during excitation. That is followed by
the photoelectron spectra of CO2, OCS,
and CS2, which were determined by
Shirley et al. on nozzle jets, and by the
classical study of NO2 ions by Leinberger et al. Next, the authors discuss
the interpretation of rotation-resolved
electron spectra of linear molecules,
taking C3 and noble gas complexes of
Mg+ as examples. NO is then discussed
as an example of an open-shell system,
in which one has to take into account the
couplings between the different angular
momentum vectors. As examples of
rotation-resolved spectra of molecules
with low symmetry, the authors discuss
the ZEKE (zero electron kinetic
energy) spectra of Al(H2O) and Al(D2O), and spectra of propynal measured by laser-induced fluorescence. Various aspects of vibration-resolved electronic spectroscopy are discussed, with
propynal, 1,4-benzodioxane, chlorobenzene, and noble gas complexes of Mg+ as
examples. In the latter two cases the
important role of spin-orbit coupling is
explained. Diphenylamine provides a
particularly striking example of the
effect of Franck–Condon factors on the
band structure. The importance of
vibronic coupling (the Herzberg-Teller
effect) is illustrated by the case of
benzene. The example of the chlorobenzene cation serves for a comparison of
ZEKE and MATI spectroscopies, in
which the advantages of the two methods are weighed against each other. In
the final example, the authors show how
cavity ringdown spectroscopy has been
used to determine absorption crosssections for the doubly forbidden
1
D X3Sg transition of O2.
In choosing their examples, the
authors have tried not only to illustrate
a wide variety of different types of
spectroscopic problems, but also to
present as many different spectroscopic
techniques as possible. Results from
modern quantum-chemical calculations
leading to vibrational frequencies, the
sequence of electronically excited states,
etc., are also included wherever possible. In some cases, the authors themselves have performed these calculations, as they were not yet possible in
this form at the time of the original
publication. The discussions and explan-
ations of the individual case studies are
clear and coherent. A possible improvement would have been to place more
emphasis on comparing and interrelating the individual examples. It is noticeable that the individual case studies are
mainly treated independently of each
other, and it seems likely that their
analysis was written at different times.
However, although that has led to a
certain amount of overlap and repetition, this could certainly be an advantage for readers who use the book as an
aid to entering the field of electronic
spectroscopy.
Considering the work as a whole, it
can be assumed that a reader who works
conscientiously through these 16 examples, and also follows up the references
to the original publications in a few
cases, will have learned a great deal
about optical spectroscopy. Nevertheless, I doubt whether that would be
possible without some previous knowledge. In my opinion, the basic principles
explained in Part 1 of the book are not
enough to provide that knowledge. Not
only is Part 1 too short for that, it is also
inadequate because of the fact that the
authors have aimed to cover the subject
with only a minimum of mathematics. It
is true that they have tried to compensate for that by including a number of
appendices, but there too the explanations are very brief. For example, in Part
1, under the heading “Electronic Structure”, the molecular Hamiltonian operator is shown as well as a wave function
that results from a product of singleparticle functions, but the discussion is
not taken any further. Appendix B
(following Appendix A, which is a
collection of constants and conversion
factors) covers the topics of antisymmetry, the Hartree–Fock method (limited
to the closed-shell case, although that is
not explicitly stated), the LCAO
approximation, and electron correlation, all within a mere seven pages. The
explanation in Part 1 of how the complete set of electronic states is built up
according to the orbital picture is limited to just two examples, H2 and NH2.
The very brief discussion of the orbital
picture is inadequate, in view of the fact
that it is used very widely throughout
the text, sometimes in an oversimplified
form. For example, on page 5 we read:
“In particular, the spectroscopic transi-
, 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6983
Books
tion energy can be equated with the
difference in energy between the two
orbitals involved in an electronic transition”.
The sections on molecular vibrations
and rotations in Part 1 are somewhat
more detailed. Although here too one
finds that energies and term values are
given without their derivations, the
information that is provided should at
least be sufficient for an understanding
of the case studies. The part on fundamentals ends with a discussion about
selection rules, with particular emphasis
on the Franck–Condon principle. Here
again, the mathematical treatment is
kept to a minimum, and consequently
the selection rules that are given have
essentially the character of recipes.
Part 1 also contains a short section
(3 pages) on angular momentum vectors, which is further supplemented by
two appendices, the first dealing with ls
and jj coupling, and the second with
Hund@s coupling cases a and b.
The frequent use of symmetry considerations, symmetry symbols, and
character tables presents a problem, as
it assumes some familiarity with these
concepts. To help readers who are
unfamiliar with that area, there is a
slightly longer appendix, which aims to
provide an introduction to the symmetry
of point groups and to group theory.
Those who already have a basic knowledge of the principles can certainly
benefit from reading this appendix.
However, whether it is sufficient as an
introduction for readers without previous knowledge is an open question.
In Part 2, “Experimental Techniques”, the authors give a short introduc-
6984
www.angewandte.org
tion to the standard methods, and also
describe a number of more recent
developments. Techniques that receive
special attention are Fourier transform
spectroscopy, continuous and pulsed
nozzle jet expansion (including possibilities for generating free radicals, ions,
and clusters), matrix isolation, and, of
course, lasers, which are today@s favored
light sources for spectroscopy. It is
regrettable that the discussion of different types of lasers does not cover diode
lasers, which are becoming increasingly
important. There are also brief descriptions of special techniques such as LIF,
double
resonance
spectroscopy,
REMPI, and cavity ringdown spectroscopy. In the context of photoelectron
spectroscopy, in addition to the usual
HeI and HeII light sources the authors
discuss the use of synchrotron radiation.
Other techniques described are Penning
ionization, and especially ZEKE
spectroscopy, which also plays an important role in some of the case studies.
However, as Part 2 is comparatively
short it cannot, of course, provide an
exhaustive treatment of the techniques
that are discussed. In view of that
limitation, it is especially pleasing that
many important topics of modern optical spectroscopy are at least touched
upon. However, I was disappointed that
the discussion of line widths did not
mention the words homogeneous and
inhomogeneous, despite the fact that the
distinction between these broadening
mechanisms is not only important in
hole-burning experiments, which are not
mentioned. With regard to the appendices, it should also be mentioned that
they refer the reader to modern compu-
, 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
tational methods and program suites.
The only recent developments in this
area that are not covered are CASPT2
and time-dependent DFT (TDDFT).
Who are the readers that the book is
intended for? It is certainly not aimed at
experts in vibration- and rotationresolved optical spectroscopy, but
rather at advanced students, and perhaps also at experimentalists who are
likely to encounter problems of this
kind. The idea of informing readers
about spectroscopy by means of case
studies has much in its favor. On the
other hand, as mentioned above, it is
doubtful whether the basic knowledge
offered within the book is sufficient to
enable students to work through the
case studies. However, the student who
already has some relevant knowledge
from lectures in physical or theoretical
chemistry, and works through the case
studies carefully, should gain a great
deal from them. From that viewpoint,
the book is certainly also useful as
material for a seminar on optical
spectroscopy. However, with a little
effort a lecturer could put together a
collection of suitable examples independently. In summary, I recommend
the book for everyone who has a basic
knowledge of spectroscopy and quantum mechanics and wishes to progress to
more advanced work in high-resolution
spectroscopy.
Georg Hohlneicher
Institut f5r Physikalische Chemie
Universit7t K8ln (Germany)
DOI: 10.1002/anie.200585293
Angew. Chem. Int. Ed. 2005, 44, 6983 – 6984
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