Table Of ContentStudies in SurfaceScienceand Catalysis 26
VIBRATIONS
AT SURFACES 1985
Proceedingsofthe Fourth InternationalConference,Bowness-on-Windermere,
United Kingdom, 15-19 September 1985
Editors
D.A. KING, N.V. RICHARDSON and S.HOLLOWAY
TheDonnan Laboratories, UniversityofLiverpool, LiverpoolL693BX, UnitedKingdom
Dedicated to ProfessorT.B. GRIMLEY
Reprinted from theJournalofElectron Spectroscopyand Related
Phenomena, Volumes 38 (Part A) and 39 (Part B)
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xv
FOREWORD
The fourth International Conference on "Vibrations at
Surfaces" was held at Bowness on Lake Windermere, England, from
15th to 19th September 1985. Previous meetings in this series
were held in Juelich, Federal Republic of Germany (1978); Namur,
Belgium (1980); and Asilomar, California, U.S.A. (1982). The
proceedings of the Third Conference were also published in the
Journal of Electron Spectroscopy and Related Phenomena, Volumes
29 and 30. The present Proceedings reflect well the significant
advances that have been made in the field of surface vibrations
since 1982, and also demonstrate a healthy tendency to develop in
new directions, particularly in relation to dynamics at surfaces.
Topicality was ensured by including postdeadline sessions, and
short communications from these authors are included in the
Proceedings.
The conference attracted 132 participants active in the field
from 14 countries, the largest representations being: United
Kingdom (39); U.S.A. (38); Germany (20); and Sweden (14). It was
organised by a local committee composed as follows:
D.A. King (Liverpool) (Chairman) N.V. Richardson (Liverpool)
B.E. Hayden (Bath) (Local Organiser)
V. Heine (Cambridge) N. Sheppard (East Anglia)
S. Holloway (Liverpool)(Secretary)R.F. Willis (Cambridge)
The committee was aided in the selection of invited speakers
by an International Advisory Committee, consisting of:
N. Avery (Australia) B. Lundqvist (Sweden)
G. Benedek (Italy) A. Luntz (USA)
J. Bertolini (France) D. Mills (USA)
V. Bortolani (Italy) H. Morawitz (USA)
A. Bradshaw (Berlin) A. Otto (W. Germany)
L. Dobryzinski (France) J. Oudar (France)
B. Feuerbacher (W. Germany) J. Pritchard (UK)
N. Garcia (Spain) J. Toennies (W. Germany)
1. Grimley (UK) H. Weinberg (USA)
H. Ibach (W. Germany) J. Yates (USA).
U. Landman (USA)
XVI
These proceedings are dedicated to Professor Tom Grimley for
his outstanding contributions to the development of theoretical
techniques in surface science in general, and to vibrations and
dynamics in particular. We therefore include in this issue, in
addition to his opening address to the meeting, a brief autobio-
graphical account. During the conference, a lively appreciation
of Tom Grimley's contributions was given by Dr. J.W. Gadzuk, who
presented him with an original painting of the English Lake
District on behalf of the conference delegates.
The attendees were in sole occupation of the Belsfield Hotel
for the duration of the conference. The siting of the Hotel, on
Lake Windermere, and the food and service (but not the weather!)
were the subject of many favourable comments. In particular,
however, it provided an ideal venue for informal discussions, and
it is hoped that this format will be continued at future meetings.
During the meeting the Parent Committee of the series (cons-
isting of past Conference Chairmen) decided that the next meeting
should be organised by Professor Alex Bradshaw (Fritz-Haber
Institute, Faradayweg 4-6, 0-1000, Berlin 33) in West Germany,
from 6th to lOth September 1987. It is anticipated that the
venue will be at a hotel location in the Bavarian Alps. The
following meeting would go to a venue in the USA or Canada in
1989.
~::i"~
;V~~~
Neville RichaJdson
~et~dJ~(
Stephen Ho Iloway
Liverpool,
November 1985
XVII
ProfessorT. B.GRIMLEY
XVIII
AUTOBIOGRAPHIC REMARKS
T.B. Grimley
The Donnan Laboratories, University of Liverpool, PO Box 147,
Liverpool L69 3BX, UK
I worked on a Surface Science problem for my PhD 40 years ago at the
University of Bristol with N.F. Matt (Now Sir Neville). Matt was interested
in the theory of the photographic process, and as part of my contribution to
the subject, I worked out the theory of the electrical double layer at the
interface between an ionic solid (AgBr), and an electrolyte. What was new in
my approach was that I assumed that the charge on the solid does not reside in
the interface in the form of adions, but as a diffuse space charge of lattice
defects (Schottky or Frenkel). Thus, the isoelectric point of the sol, and,
for example, the variation of the electrokinetic ~-potential with P were
Ag,
related to the parameters determining the defect concentration in the solid.
It was as a direct result of this theoretical work that I went to the
University of Liverpool where there was at that time (1949) a research interest
in the lyophobic colloids. However, I developed another aspect of my PhD work;
the electronic structure and bonding in partly covalent crystals like the new
halides. Through this work I began to understand the complex problems of
bonding in solids and in molecules, and came to experience what C.R. Burch
called "the broadening effects of specialisation".*
About 1950, C.E.H. Bawn drew my attention to the poor state of the theory
dilute solutions of high polymers, and in the next 10years I published 7
papers, and a book chapter on this subject. However, it was during the last
half of this decade that the problems in Surface Science began to occupy more
of my time. W.E. Garner asked me to write the chapter on "Oxidation of Metals"
for his book "Chemistry of the Solid State", published in 1955. Although I
had not published on the subject, Garner arguedthat I had worked with N.F.Mott
(on a different subject!) at the time when N. Cabrera was collaborating with
him on the theory of metal oxidation, so I must know a lot about the subject,
and since I was now in a Chemistry Department, I would know how to present the
subject to chemists. I accepted this curious argument, and shortly afterwards
Garner invited me to contribute a paper on "Chemisorption Theory" to a
Symposium on Chemisorption organised by the Chemical Society (now the Royal
Society of Chemistry) at Keele in 1956. Again, I had not at that time
published anything on chemisorption. My paper contained a treatment on hydrogen
*By seeking to understand one thing, you come to know about, and to understand
many other things, most of which you had not even heard of before your enquiry
started. In a different field "The Golden Bough" by Sir James Frazer is a
good example.
XIX
chemisorption by cubium in the tight-binding (H~ckel) approximation, and
showed how localized electronic states above, or below, the cubium band might
be formed. J. Koutecky was also investigating these "chemisorption states"
at the same time.
My major paper on the subject using Green function techniques was not
published until 2 years later (1958). At that time I concentrated on adsorbate-
induced localised states because much of the experimental data of chemisorption,
and catalysis could be interpreted most easily in terms of the existence of
surface compounds, complexes or molecules, with properties similar to gas-phase
counterparts, and therefore by inference, with chemisorption bonds similar to
those in ordinary molecules; evidently moleCUlar orbitals (MO's) localised on
the adsorbate, and a few substrate atoms near it would lead to a localised
chemisorption bond of the required type. However, I soon came to doubt that such
localised MO's could be a sufficiently widespread phenomenon, and in my
review article for Advances in Catalysis XII published in 1960, I showed how
the chemisorption bond could be formed to a metal substrate without localised
MO's, but with every itinerant electron in the system contributing to the bond.
However, lacking self-consistent calculations for real systems, I did not feel
that further discussion of this matter would be profitable, and for a time I
worked on other problems (high polymer solutions, Markoff chains, metal oxidation,
Fermion density matrices). Of course, we now know that chemisorption is a spatially
localised phenomenon if the adsorbate-induced change in the local density of
states on a substrate atom is significant only for a few substrate atoms near the
adsorbate. This condition is certainly met if the important adsorbate-induced
electronic structure consists of localised MO's, but the fact that it can be
met when no such MO's are present, is the reason why localised chemisorption
is such a widespread phenomenon.
In 1961 I became interested in the mechanical renormalization of the
surface bond vibration frequency. I hadmet R.P. Eischens early that year,
and I wondered why it was possible to ignore the substrate phonons in inter-
preting the IR spectra of chemisorbed molecules. This problem was interesting
theoretically becauseof its structure; a system of particles (the molecular
modes) interacting with a field (the substrate phonons). Such problems were
becoming well understood in several branches of theoretical physics, and it
was known that, as a result of the field-particle interaction, a certain
renormalization of the particle parameters would occur. My problem was to find
out how large the frequency renormalization (actually on increase) would be
for a real system (H or CO on platinum for example). I found it to be negligible,
except for a system with a molecular mode lying rather close to the phonon band
xx
edge, and this is why the IR spectra of chemisorbed molecules can usually be interpreted
in a straightforward way.
I returned to Surface Science in 1966 when I was approached by P. Debye
to participate in the Study Week onMolecular Forces organised by the Pontifical
Academy of Sciences, and held in the Vatican City. As there would be experts onvan
der Waals forces present (H.G.B. Casimir, J.D. Hirschfelder), I decided to use the
field-particle formalism to compare the indirect (electron-mediated) interaction
between molecules chemisorbed bymetals, with the retarded van der Waals interaction
between gas-phase molecules. Both J. Koutecky and I had drawn attention to this
interaction some 8 years earlier, but the law of force was unknown. Now, by using a
trivial generalization of P.W. Anderson's Hamiltonian to describe chemisorption, and
Green function techniques, the law of force, and other essential features of the
electron-mediated interaction were rather easily discovered.
Anderson's Hamiltonian, though invaluable for qualitative work in
chemisorption, is inadequate for quantitative work on real systems (it was after
all designed to deal with a quite different problem), and in a lecture to the
Battelle Colloquium held in Kronberg in 1968, I explored two different approaches
to CO chemisorption by transition metals. In one, I applied R.S. Mulliken's
discussion ofdonor-acceptorcomplexes to chemisorption; in the other I introduced
the notion of a surface compound as a small adsorbate-metal cluster in communication
with the rest of which determines, amongst other things, its Fermi level. Asurface
compound therefore contains in general, a non-integral number of electrons to be
determined by a self-consistent calculation. I performed this "self-consistent
embedding" in a simple way for CO on nickel. The first approach (the donor-acceptor
approach) leads directly to the generalization of the frontier orbital discussion of
symmetry factors in chemical reactions, and enables us to anSwer the question of
whether there are symmetry factors of the Woodward-Hoffman type operating in
chemisorption. Formetal substrates, in general there are not, but there is still
some confusion in the literature so only this year at the suggestion of N.V. Richardson,
I wrotethe computer programs to verify this for NO decomposing on a tight-binding
tungsten slab.
The full computation in the Hartree-Fock (HF) approximation of the
electronic structure, and total energy of a surface compound, i.e. of an embedded
cluster, seemed tome to provide the way forward to quantitative theoretical work
on chemisorption, but I was not at that time capable of writing the large-scale
computer programs to do it. Instead I explored the various aspects of chemisorption
using free clusters, and model Hamiltonians (Anderson's and Hubbard's) for which
exact results could be obtained with relatively little programming effort. My work
with B.J. Thorpe, and M. Torrini was of this sort. I also showed how the over-
completeness of the basis in Anderson's Hamiltonian, which causes computational
problems, could be overcome.
XXI
Of course, not every investigation leads to a publication, and as an
example, I mention that about this time (1971-72) I tried to study the Mott
transition by computing exactly the ground, and first excited states of small
numbers of atoms (chains, rings, Cayley trees) described by Hubbard's Hamiltonian.
I found the few lowest eigenvalues, and eigenvectors of matrices up to 4900 x 4900
using sparse matrix techniques of course, but as this large matrix described only
8 hydrogen atoms I was not able to ascertain the behaviour (gap or no gap) for a
large enough number of atoms.
By the early 1970's I was writing down the general equations of
chemisorption theory using a localised (atomic orbital) basis set, and at the Varenna
Summer School of 1973, I gave the equations for treating Hatom chemisorption on
a tight-binding solid by using F.J. Dyson'S equation to embed a 2-atom cluster
(hydrogen + one substrate atom) in the rest of the substrate. The same year
C. Pisani spent 6 months with me at Liverpool, and wrote the computer programs
for this self-consistent embedding in the HF approximation so that in 1974 we
published the first LCAO-MO computation of hydrogen chemisorption on a semi-
infinite substrate. Nevertheless, even now 11 years later, there are computat-
ional problems still to be solved before this type of calculation will iterate
to self-consistency for an arbitrary adsorbate/substrate systerr,. But already
in th~ mid-1970's one could say that chemisorption theory had become a problem
in computational chemistry. The surface bond was understood in the sense that
we could see how a localised chemical bond could be formed to a substrate which
didnot itself have localised orbitals. Therefore I began thinking more about
dynamical processes at surfaces, and at the Faraday DiscusSion of the Chemical
Society at Cambridge in 1974, I discussed various aspects of photoemission,
photodesorption, and photochemistry of adsorbates. The following year at the
Battelle Colloquium held in Gstaad, I emphasized the importance to heterogeneous
catalysis of theoretical studies of the dync~ics of elementary processes at solid
surfaces.
I worked on adsorbate-induced photoemission between 1974 and
1977 with a view to obtaining essentially exact results fer some simple models.
When a subject is in its initial stages of development, there are advantages
in having available the exact solution to a simple, but non-trivial model, and
with G. Doyen, I made mocel calculations for Hon copperwhich illustrated much
of the formal theory, and provided examples of relaxation, shake-up, plasmon
satellites, sum rules, electron correlation effects, and so on. Alittle earlier
G.F. Bernascani and I had computed the angle andenergy resolved photoemission
into plane wave final states from hydrogen o~ cubium, and on lithium using the
embedded HF cluster theory of chemisorption which I had developed with C. Pisani
to provide the Green function matrix which characterizes the initial states.
XXII
These computations, published in 1975, amply demonstrated the power of UPS
to provide information on the clean-surface crystallography, and on the
symmetry of the adsorption site.
Between 1976 and 1978 I worked on the computation of core hole spectra
of adsorbates. With J.A. Sanches, I used once more the embedded HF cluster
theory to treat the chemisorptionof Li atoms by cubiums. Then, at time t = 0
the core hole was created, and by numerically integrating the differential
equation for the time evolution operator, we followed the time development of
the initial state. The spectrum was obtained by taking the Fourier transform.
We also made delta-HF computations on small clusters, but the time-evolution
method is a perfectly feasible way to compute the core hole spectrum of an
adsorbate on a semi substrate.
7infinite
In 1975 there began a colloboration with G.P. Brivio which continues to
this day, and which has concentrated on dynamical problems. I had raised the
question of non-adiabatic processes at the Gstaad Colloquium, and the same
year (1975) I started work on the full quantum mechanical treatment of the damping
of adsorbate vibrations (IR linewidth) and translations (sticking) with energy
transfer to electron-hole pairs. The situation of most interest for electron-
hole pair generation is a light atom forming a strong chemisorption bond to a
metal, and we have this year published exact results for the sticking coefficient
of such a system, using of course, a rather simple Hamiltonian to describe the
dynamics. We had already in 1976 computed the IR linewidth for the same system
giving a result ( 2 cm ) similar to that subsequently observed for CO on
(Ill) Pt by D.A. King.
To compute these dynamical processes involving electron-hole pair
excitations, one has first to solve the chemisorption problem to obtain the
"static input" to the dynamical problem. It was because I had all the necessary
static input for hydrogen on cubium from my work with C. Pisani in 1973-74
that I was able to go beyond formal equations, and to compute numbers for
dynamical processes - albeit for a simple theoretical model. I do not think
I would havepublished on process dynamics otherwise. When I started research
40 years ago things were different. You could derive some equations, and
explore SomE' limiting cases using Taylor'S theorem, or the method of steepest
descents, and publish a paper. Nowadays you have to make comprehensive
computer calculations for some theoretical model, and those of my age who
have not taken to computing, have, in general, less influence than those
who have.
My work with G.P. Brivio treats everything quantum rrechanically,
but there are many gas-solid collision processes where it is sufficient
to treat the motion of the gas atom classically. Perhaps the simplest example
is the electron transfer between a solid, and a fast (500 eV) atom or ion
reflected from it. The gas atom's classical trajectory defines a time-dependent.
