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OCR for page 144
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Surfaces en c! Interfaces
INTRODUCTION
The outermost layer of atoms in the surface of a crystal has been
studied for over five decades, since the observation of diffraction of
electrons by the two-dimensional array of atoms in the surface of a
nickel crystal established the wave nature of the electron in a clear and
unambiguous fashion. For years, the field was plagued by the inability
to prepare surfaces sufficiently clean and well characterized to ensure
the reproducibility of data. This problem was solved by the develop-
ment of ultrahigh-vacuum techniques during the past two decades.
Now, when a surface is prepared, it can be maintained perfectly clean
for 1 hour or longer, while a variety of precise measurements are
performed on it.
In the past 10 years many new experimental probes have been used
for the study of the structural and dynamical properties of the crystal
surface and of atoms or molecules adsorbed onto it. These new probes
are summarized in Table 7.1. In parallel with these developments,
there have been rapid advances in surface-physics theory. Numerous
examples now exist where important new conclusions have followed
from the interplay between theory and experiment. In addition, we see
substantial progress in the development of both ab initio descriptions
of the electronic structure of clean and adsorbate-covered surfaces and
of the dynamics of crystal surfaces.
144
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S UREA CES A ND I N TERFA CES 145
TABLE 7.1 Experimental Techniques Used in the Study of
Physical Properties of Surfaces and Interfaces
Physical Property Studied
Elementary Interface
Excitations Atoms and Between
on the Molecules on Solids and
Experimental Technique Structure Surface the Surface Dense Media
Ion beams
Raman spectroscopy
Scanning vacuum tunneling X
X
X X X
microscope
Synchrotron radiation X X
Electron energy-loss X X
spectroscopy
Electron microscopy X
Atom/surface scattering X X X
Low-energy electron X X
diffraction
Neutron scattering X X X
X rays X
Infrared spectroscopy X X X
Spin-dependent electron X
scattering
Brillouin scattering X X
Diffusion of adsorbed X
species
Molecular beams X
Laser-induced desorption X
or fluorescence
Inelastic electron-tunneling X
spectroscopy
This chapter is concerned primarily with the physics of the outer-
most atomic layer or two of single crystals in an ultrahigh-vacuum
environment, along with that of monolayer quantities of adsorbate
atoms or molecules upon it. The adsorbates may be chemisorbed, i.e.,
bound to the surface tightly via chemical bonds similar to those
encountered in molecules, or physisorbed, where only a much weaker
van der Waals attraction traps the adsorbate near the surface. A1SO7 a
substantial interest exists in the microscopic nature of the solid-gas or
the solid-liquid interface, where the first few layers in the low-density
phase may have properties modified profoundly by their proximity to
the solid interface. Some of the new methods of studying the interface
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146 A DECADE OF CONDENSED-MATTER PHYSICS
between a crystal and vacuum are directly applicable to the analysis of
the liquid-solid or the gas-solid interface, as we shall see.
If one considers the outermost atomic layer of a perfect crystal, one
may inquire whether it is a replica of a plane of bulk atoms or whether
it differs importantly. The latter is frequently the case. The atoms may
shift off the sites expected from the bulk structure to form a new,
low-symmetry phase unique to the surface. This is known as surface
reconstruction and is observed on many surfaces. The electronic
structure of the surface may be unique, because of unsaturated
dangling bonds. Such electronic states are often responsible for the
particular chemical reactivity of the crystal surface.
Adsorbate overlayers are a rich area for study. The environment of
a chemisorbed species differs greatly from that in an isolated molecule,
and this leads to new electronic configurations. At finite coverage,
adsorbates may interact directly via the overlap of their wave func-
tions, indirectly via the perturbation of the electronic structure of the
substrate, or by local strains induced by chemisorption. Such lateral
interactions control ordering of the overlayers, and thus control the
thermodynamic phase diagram of the adsorbate/substrate combination.
Physisorbed rare-gas atoms are weakly bound to the surface and may
move parallel to the surface relatively unhindered. One may view the
substrate as a passive entity whose role is to confine the atoms to a
plane parallel to its surface; hence the absorbate system constitutes a
realization of two-dimensional matter. From recent theory we know
that physics in two dimensions differs profoundly from that in three
dimensions. Hence, study of physisorbed overlayers offers insight into
basic issues of statistical mechanics in two dimensions.
We have outlined why, from the point of view of fundamental
physics, the study of surfaces is of great interest. In addition, advances
in our understanding of surface physics have a direct impact on other
areas of science. In the chemical industry, solid-state catalysts are
extremely important in many manufacturing processes. While a par-
ticular catalyst often proves highly efficient for a limited range of
reactions, little is understood about the origin of this specificity.
Knowledge of the basis for efficient catalytic activity will allow the
design of new catalytic structures. Practical catalysts are typically
complex, multicomponent systems, prepared in powder form, and
operating in a high-pressure environment; they differ substantially from
a single-crystal surface, prepared and cleaned in ultrahigh vacuum.
However, the fundamental principles elucidated in the study of the
single-crystal surface and its interaction with atoms or molecules will
form the basis of a deeper understanding of how real catalysts function.
Also, diagnostic methods developed in surface physics have been
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SURFACES AND INTERFACES 147
applied to the analysis of real catalysts. In the past 3 years, a com-
bination of surface-physics methods (Auger spectroscopy, low-energy
electron diffraction, and electron energy-loss spectroscopy) has been
used to study the decomposition of hydrocarbons on single-crystal
platinum surfaces. This has allowed us to trace out, in a step-by-step
fashion, how their decomposition leads to formation of a carbonaceous
layer, and the consequent poisoning of the surface as a catalyst.
In materials science, there is great interest in new multilayered
structures formed by deposition of two or more materials and consti-
tuting a macroscopic material with unique properties. In idealized
form, such a structure consists of single-crystal films, with the thick-
ness of each controlled and in the range of a few tens to a few hundreds
of angstroms. Semiconductor superlattices are one example of such
materials. There are also new superlattices formed from metals or from
combinations of (ferromagnetic) metals and semiconductors. The study
of the single-crystal surface, and of adsorbates on it, may be viewed as
the study of the first atomic layer of a new constituent in a superlattice
structure. Thus, research in surface physics has a direct impact on this
exciting new area of materials science.
THE STRUCTURE OF THE CRYSTAL SURFACE
If one is to understand properties of, and bonding to, the outermost
layer of a crystal, a first step is the elucidation of the geometrical
arrangement of the constituents. Thus, considerable effort is spent on
the development of probes that provide structural data and on theo-
retical descriptions of the interaction of the probe with the surface.
To extract structural information from the data is formidable. The
problem is that the probe must either reflect off the outermost layer or
backreflect after penetrating only a small number of layers. This means
that it interacts strongly with the crystal. Hence full utilization of
information in the data requires a sophisticated theory that treats the
strongly interacting probe completely. (In contrast, in the study of
bulk structures, the probing quanta x rays or neutrons, for ex-
ample-travel long distances in the crystal; their interaction with any
one constituent is weak. This leads to simple theories for interpreting
data.)
It proves difficult to determine a surface structure unambiguously
from one set of data. Consequently, several methods are often used to
study a single structure. There is no single probe or method, such as
x-ray scattering used in bulk studies, that solves surface-structure
problems.
Many surface structure studies employ either low-energy electron
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148 A DECADE OF CONDENSED-MATTER PHYSICS
beams, with energy in the range of a few to a few hundred electron
volts, or neutral atom or ion beams. The electrons sample a small
number of layers, three or four in number, while neutral atom beams
sample only the outermost contours of the electron charge density.
Both have a deBroglie wavelength comparable to lattice spacings, or
bond lengths, and thus serve as sensitive probes of microscopic aspects
of surface geometry.
Of the electron spectroscopies. low-energy electron diffraction
(LEED) has been a mainstay for many years. Through its use one may
identify systematic structural trends. Recently it has been established
that on most metal surfaces there is a contraction between the first and
second layer, with the more open faces contracted the greatest.
Observations such as these have provided a major stimulus to theory.
A major development of the past decade is the utilization of
synchrotrons as intense sources of electromagnetic radiation with a
continuous spectrum of wavelengths, which extends from the visible,
through the ultraviolet, and into the x-ray region. Many new surface-
sensitive spectroscopies based on these sources have emerged during
the past decade. Among them photoemission has developed rapidly.
Here a photon, which penetrates many atomic layers, will eject an
electron from the crystal. One measures the total current emitted by
the crystal, the energy distribution of the emitted electrons, or their
angular distribution. Sensitivity to the surface arises because the mean
free path of the excited electron is a few lattice constants. Thus, only
those excited close to the surface emerge.
Ultraviolet photons excite electrons from the valence orbitals of the
atomic constituents of crystals. One finds here surface electronic states
revealed by photoemission; these states are markedly affected by the
surface geometry, so their spectroscopy provides important informa-
tion on surface structure. A decade ago, we had few data in hand in this
area, but now the influence of a surface on the electronic structure of
materials has been explored experimentally for many semiconductors
and many metals.
Other spectroscopies employ synchrotron radiation as the basic
probe. A photon may eject a core electron from an adsorbed atom or
molecule. The emitted electron wave propagates to the detector, at the
same time that a backscattered portion reflects off the surrounding
structure to interfere with the direct wave. Study of the energy and
angular variation of the cross section for this process provides infor-
mation on the local environment of the atom or molecule involved.
X-ray absorption edges exhibit fine structure, with a closely related
origin. Since the synchrotron's output is in the form of a broad,
continuous band of radiation, these features may be explored in detail.
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S UREA CkS A ND /NTERFA CkS 1 49
New areas of synchrotron-radiation-based spectroscopy are in early
stages of development. For example, while x rays have not generally
been considered a surface-sensitive probe, at glancing incidence their
electromagnetic fields are evanescent in the substrate. Thus, the Bragg
beams reflected from the crystal contain information about the outer-
most region of the crystal. In particular, if the surface reconstructs,
new Bragg beams are induced by the reconstruction process. These
beams provide information about the magnitude of the atomic displace-
ments parallel to the surface. Recently, more complete analyses have
provided information on the vertical displacement of atoms. The
method may be unique in providing access to displacements associated
with reconstruction, without recourse to the theories required in
electron- or atom-beam studies.
Glancing-incidence neutron spectroscopy also offers the possibility
of surface sensitivity, though the method has yet to be implemented
fully.
One exciting new probe has appeared recently: the tunneling elec-
tron microscope. This device operates on the basis of a fundamentally
new principle. If two metals are placed ten or so angstroms apart, with
a potential difference between them, an electron may transfer from one
to the other via quantum-mechanical tunneling. One metal is a sharp
tip, while the second is the sample to be studied and is nominally flat.
The current that flows is sensitive to the distance between the tip and
the sample; if the tip is scanned across the sample, the current will
fluctuate in magnitude in response to protrusions on the surface that
change the distance from the probe tip to the sample. Features in the
surface profile that influence the current are steps or defects on a length
scale of a few to a few tens of angstroms, or possibly the bumps in the
electron density produced by the individual atomic constituents. A
direct, real space map of the surface geometry is produced.
The current spatial resolution of the device renders large-scale
surface features, such as steps, readily visible. Since steps and other
defects on the surface play a major role in catalysis, in the nucleation
of reconstructed phases, and in other surface processes, this is an
exciting development. The spatial resolution allows studies of atomic
arrangements in open surface structures. Thus, the 7 x 7 reconstruc-
tion of the ( 1 1 1 ) surface of silicon has been probed directly (Figure 7. 1),
as has the (110) surface of gold.
As remarked earlier, low-energy atoms have a de Broglie wavelength
comparable to crystalline lattice constants, and such atoms are
backscattered from the outermost portions of the electron charge-
density contours. Recently there has been a great improvement in our
ability to prepare monoenergetic beams of light atoms, such as He. At
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150 A DECADE OF CONDENSED-MATTER PHYSICS
FIGURE 7.1 Relief of two complete 7 x 7 unit cells on a reconstructed silicon (111)
surface, with nine minima and twelve maxima each, taken at 300°C. Heights are
enhanced by 55 percent; the hill at the right grows to a maximal height of 15 A. The (21 1 )
direction points from right to left, along the diagonal. (Courtesy of H. Rohrer, IBM
Research Laboratory, Zurich.)
present, beams with energy spreads well below 0.1 meV are readily
available. The study of elastic- or Bragg-scattered beams is rich, with
scattering resonances evident that are associated with bound states of
the atom/surface interaction potential. These are sensitive to the details
of the potential, including its variation in the two dimensions parallel to
the sample surface.
New high-energy ion backscattering studies place important con-
straints on surface geometry and provide quantitative information on
whether surface atoms are shifted off high symmetry sites or if there is
a contraction or expansion of the distance between outermost layers of
the crystal. Here classical trajectory analyses are sufficient to analyze
the data. A set of qualitative concepts based on the shadowing of
interior atoms by those in the surface has developed that may be used
OCR for page 151
S UREA CES A ND INTERFA CES 1 5 1
to interpret data. This is an experimental technique that will continue
to be developed in the coming years.
Electron microscopy is another technique emerging as a surface
probe of substantial importance. Transmission of electrons through
thin films offers the possibility of studying electron diffraction under
conditions where a single scattering description is appropriate. A
dark-field imaging method can pick out only superlattice reflections and
thus can be used to examine the dynamics of formation of the (7 x 7)
structure on the Si ( 1 1 1 ) surface. It is even possible to observe
nucleation of the new phase near steps on the surface with this
technique. Electron microscopy will become an important tool for
exploring a range of issues such as the nature of defects on the surface,
dynamical aspects of the surface environment, and also microscopic
aspects of surface structure.
A final probe that is being used extensively in surface studies is the
field ion microscope. It employs a needle-shaped specimen with an
electric field at its tip so strong that inert gas atoms are ionized; the ions
subsequently follow the electrostatic field to an imaging screen. Since
the fields are strongest at the surface and are sensitive to surface
features on an atomic scale, the pattern of ion impacts on the screen
images atomic details of the sample tip. Surface structure, atomic
diffusion on surfaces (Figure 7.2), and chemical groupings of adatoms
are the types of surface properties that are now investigated In a direct
way by field ion microscopy. In addition, however, by means of strong
field pulses it is possible to strip away successive surface atoms and
reveal underlying structure. This makes depth profiling a feasible, if
tedious, procedure. When coupled with a time-of-flight mass spectrom-
eter the field ion microscope thus permits a structural and chemical
map of the tip surface and underlayers to be built up sequentially at
atomic resolution.
The past decade has clearly been one in which classical methods of
deducing surface geometry have advanced in a qualitative manner,
while a number of new and potentially powerful methods of analysis
are under active development.
SPECTROSCOPY AND ELEMENTARY EXCITATIONS ON THE
SURFACE
A major advance in understanding the physics of bulk solids oc-
curred when inelastic neutron scattering became widespread. The
neutron may scatter inelastically oD of any elementary excitation such
as a spin wave or phonon, and an analysis of the kinematics of the
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152 A DECADE OF CONDENSED-MATTER PHYSICS
FIGURE 7.2 Movement of a single Re adatom didusing on the central (21 I) plane (dark
circular area) of a tungsten crystal field ion microscope tip. The four images taken at
successive times show the Re adatom progressively displaced. (Courtesy of K. Stolt and
G. Ehrlich, University of Illinois.)
scattering event allows one to map out their dispersion curves. Since
the de Broglie wavelength of thermal neutrons is of the order of a
lattice constant, one may study the dispersion relations throughout the
entire Brillouin zone. Such data have led to a qualitative expansion in
our understanding of perfect and imperfect crystals.
During the past 2 years, two methods-inelastic scattering of helium
atoms and electron energy-loss spectroscopy have been developed to
the point where they can now be used for detailed measurements of
surface phonon dispersion curves.
In the previous section we noted that we now have in hand highly
monoenergetic beams of slow, neutral He atoms. These not only allow
one to study fine detail in the elastic cross section but have led to the
realization of high-resolution inelastic scattering studies of surface
phonons. We now have data available for several insulating and
metallic surfaces.
Shortly before 1970, electron energy-loss spectroscopy was used to
study high-frequency vibrational motions of light adsorbates, and in
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SURFACES AND INTERFACES 153
1970 the method was used to study vibrational spectra of clean
surfaces. This technique has developed into one of the standard
working tools of surface science during the past decade. As the surface
atoms or adsorbates vibrate, oscillating electric dipole moments asso-
ciated with the motion lead to intense inelastic scattering of the
electron through small angles. Virtually all of the experiments study
these near-specular losses. In this configuration one studies only
modes whose wave vector lies close to the center of the corresponding
two-dimensional Brillouin zone.
During the past S years, the first studies of electrons that suffer
inelastic scattering through large angles have appeared. A selection
rule that applies to near-specular scattering breaks down in the large-
angle studies, with the consequence that many more modes can be
explored. The first experiments explored the vibrational spectrum of ad-
sorbates. When off-specular spectra are combined with the near-specu-
lar data, a rather complete picture of the adsorbate geometry emerges.
Within the past year off-specular electron energy-loss studies have
been employed to obtain surface phonon dispersion curves for a clean
surface and also for a surface covered with an ordered adsorbate layer.
We thus have two methods that may be utilized to explore surface
phonon dispersion curves, and the coming years should prove to be an
exciting era in the spectroscopy of surface vibrations. It should be
remarked that the two methods discussed above will surely emerge as
complementary approaches to the problem.
One area of surface spectroscopy in an early stage of development,
with great future promise, is the spin-dependent scattering of electrons
from a surface. One may now produce beams of spin-polarized
electrons, through the use of GaAs emitters appropriately pumped with
laser radiation. With these beams, elastic scattering data are obtained
from ferromagnetic substrates. One may align the spin of the beam
electrons either parallel or antiparallel to the substrate magnetization
and detect the difference in scattering intensity. It is believed that this
difference is in essence proportional to the magnetization in the crystal
surface. The new data have been used to infer the temperature
variation of the magnetization in a ferromagnet below the bulk Curie
temperature and to provide the first results for this quantity close to the
Curie temperature.
Neither inelastic atom/surface scattering nor electron energy-loss
studies have provided information on the linewidths of the various
modes that have been probed. It is unlikely that the electron energy-
loss method will achieve sufficient resolution in the near future to
generate such information, and while atom/surface studies may be able
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154 A DECADE OF CONDENSED-MATTER PHYSICS
to realize sufficient resolution to measure linewidths in favorable cases,
this has not been done as yet. However, both infrared and Raman
spectroscopy offer high resolution' when applied to the study of bulk
excitation spectra of condensed matter, and one may look toward
either of these as possible high-resolution probes of surface vibrations.
Both suffer from weak signals, a disadvantage if the aim is to study
monolayer or submonolayer quantities of material on the surface of a
single crystal. Despite this basic difficulty, recent years have seen
substantial progress in each area.
There have been impressive new methods in infrared studies of
adsorbate vibrations. In one version of this method the frequency of a
surface electromagnetic wave (surface polariton) is swept through a
vibrational resonance of the adsorbate and the resulting attenuation
studied. One may achieve an appreciable enhancement here over the
signal level expected in a one-bounce reflection experiment. A second,
multiple-reflection, method in which the sample serves as a waveguide
has been utilized to obtain beautiful high-resolution spectra of
submonolayer quantities of hydrogen on a silicon surface. Finally, the
thermal emission of a warm sample placed in a cryogenic environment
has been employed to explore the vibrational motion of CO on a
single-crystal Ni surface. In all these cases, the data yield the linewidth
of the normal mode explored. These experiments employ state-of-the-
art equipment of a sophisticated nature. Since infrared spectroscopy is
at the moment the only experimental method that has provided data on
the intrinsic linewidths of simple adsorbates on single-crystal surfaces,
further application of these new approaches to a broader class of
systems is of paramount importance.
Great excitement was generated a few years ago by the discovery of
giant Raman signals from adsorbed molecules. The effect was discov-
ered by electrochemists, who were exploring the interface between an
electrolytic liquid and a silver electrode. The Raman cross section per
adsorbed molecule was found to be larger than that observed in the gas
or liquid phase by a factor of 105 to 106. This enhancement, if present
for a wide range of adsorbate/substrate combinations, would render Ra-
man spectroscopy a viable high-resolution probe of surface vibrations.
An essential element necessary for the giant signals is the presence
of roughness on the silver surface. The physical picture that emerges is
that such roughness couples the incident photon to surface electronic
resonances, whose origin may be the protrusions themselves. Excita-
tion of surface resonances enhance the electromagnetic field of the
incoming photon, and since the Raman cross section scales as the
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SURFACES AND INTERFACES 155
fourth power of the field strength, a modest enhancement in the field
leads to a large enhancement in the signal realized.
Since roughness plays an essential role in generating the giant Raman
intensities, quite clearly this form of spectroscopy is difficult to apply
to high-quality single-crystal surfaces. Also, the surface resonances
must lie in the visible, and the damping that they experience must be
modest. These two requirements limit the effect to only a limited
number of substrates, with silver a particularly favorable material.
Systematic control of the enhancement effect can be achieved utilizing
a surface on which a diffraction grating is present. The resulting
enhanced fields can be exploited to enhance a wide variety of linear and
nonlinear optical interactions near surfaces and interfaces, in addition
to the Raman effect. For example, the intensity of second-harmonic
radiation obtained from an illuminated metal surface can be enhanced
by a factor of 104, through use of surface roughness.
It is quite possible to detect Raman signals from adsorbed molecules
in the absence of the giant enhancement discussed above. While the
signals are weak, spectra from monolayer quantities of molecules
adsorbed on high-quality, single-crystal surfaces have been observed.
Raman, like infrared, spectroscopy can be carried out in a high-pres-
sure environment or can be used to explore the liquid-solid interface,
if the liquid is transparent. Thus, development of either tool to the
point where it may be used to explore a diverse range of systems will
be an important step.
A third optical spectroscopic technique that has experienced sub-
stantial development is Brillouin scattering of light from surface
phonons, surface spin waves, and other surface excitations. One may
now explore backscattering of light from metals where the skin depth
is only 150 A. Kinematical considerations restrict its application to
waves with very long wavelengths (typically a few thousand ang-
stroms), so the technique is not a microscopic probe of the surface.
However, one can measure elastic and spin-wave stiffness constants in
thin films and explore the influence of a surface on the acoustic or
magnetic response of the medium.
INTERACTIONS OF ATOMS AND MOLECULES ON THE
SURFACE
.
The understanding of the physical origin of various features in the
. .
atom/surface interaction potential will clarify many aspects of surface
chemistry. One source of quantitative information on the atom/surface
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156 A DECADE OF CONDENSED-MATTER PHYSICS
interaction potential is the analysis of the intensities of atoms scattered
elastically from crystal surfaces. As mentioned earlier, recent ad-
vances allow the preparation of highly monoenergetic, highly colli-
mated atom beams. The resulting improvement in the quality of
scattering data is truly impressive, and the theory of atom scattering
from a rigid, possibly deeply corrugated, surface has undergone
considerable development in the same period of time. A consequence
is that semiempirical interaction potentials have been constructed that
describe the interaction of rare-gas atoms with several surfaces.
There are two distinct limiting cases in the discussion of rare-gas
atom/surface interactions. A surface of an insulating or semiconducting
material is highly corrugated, and one must understand in detail the
magnitude of this corrugation whose influence on many aspects of atom
motion on or near the surface is a crucial consideration. On the other
hand, the low-index surfaces of simple metals are smooth, so the
rare-gas atoms are trapped on the surface, but they are free to move
parallel to the surface impeded only modestly by the corrugation. The
physics of overlayers adsorbed at finite coverage is then dominated by
lateral interactions between the adsorbate atoms.
In recent years, much has been learned of the physical nature of
these lateral interactions. It is now clear that the van der Waals
interactions are influenced importantly by the close proximity of the
atoms to the surface. Much attention has been devoted to the phase
diagram of monolayer or near-monolayer quantities of rare-gas adsor-
bates, in the limit where the influence of the corrugations in the
substrate potential may be ignored. The phase diagram of chemisorbed
systems has also been studied both experimentally and theoretically,
and the strengths of the various lateral interactions deduced from it.
Information on the atom/surface potential can be obtained from
experiments other than scattering experiments. The primary mecha-
nism for the diffusion of atoms over a surface is thermal activation over
the barrier between the initial and final site. This activation energy
provides information about saddle points in the potential energy
surface. Computer simulations of such dynamical processes are impor-
tant to carry out, since they can test whether a given semiempirical
atom/surface potential provides results that fit the data.
The interaction of molecular beams with surfaces has been studied
actively and will yield important results in the near future. Small
diatomic molecules can impact the surface, and a new element in such
scattering studies is the presence of the rotational and vibrational
degrees of freedom in the molecule. The molecule may now scatter off
the surface with a change of rotational quantum number, and in fact the
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S UREA CES A ND INTERFA CES 157
molecule-surface interaction potential itself may depend on the rota-
tional quantum state of an incoming molecule or of one adsorbed on the
surface. Recent experiments that study the scattering of H2 molecules
from an Ag surface show that the positions of the fine-structure
resonances, and hence the bound-state energies of the molecules,
depend on the rotational quantum number.
Lasers have also found applications in the study of various proper-
ties of atoms and molecules on solid surfaces. In the past few years,
nonlinear optical techniques have been exploited to probe surfaces and
interfaces. Second-harmonic generation in reflection from a surface has
been shown to have enough sensitivity to detect submonolayers of
atomic and molecular adsorbates. The technique is versatile. It can
yield information about the dynamics of molecular adsorption and
Resorption, the changing of adsorption sites, the spectrum of adsorbed
molecules, and the arrangement and orientation of adsorbates, for
example. That it can be extended to the infrared region makes high-
resolution vibrational spectroscopy of adsorbed molecules also a possi-
bility. Several other laser surface probes have also been developed
recently.
One may learn much about the kinetics of molecules on the surface
by analyzing the population of various rotational and vibrational states
of species desorbed or scattered from the surface; here laser-induced
Resorption or fluorescence and photoacoustic spectroscopy may be
useful probes. When one combines such information with theory
generated by computer simulations based on molecular-dynamics
routines, one gains considerable insight into those aspects of the
dynamics of molecule/surface interactions crucial to the understanding
of chemical interactions on surfaces. One may monitor the rotational
and vibrational statistics of species that come off the surface. A
non-Boltzmann distribution is frequently found, and there has been
considerable success in the comparison between such data and theory
based on computer simulations.
THE INTERFACE BETWEEN SOLIDS AND DENSE MEDIA
In many instances, one is interested in surfaces placed in a high-pres-
sure environment or in the interface between a surface and a dense
medium such as a liquid. Catalysts operate in a high-pressure environ-
ment. In electrolytic cells, a dense fluid overlies the interface.
In such systems, many experimental methods used in ultrahigh-
vacuum environments are inapplicable. Any method based on the use
of electron beams, or of atoms or ions, fails because the mean free path
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158 A DECADE OF CONDENSED-MATTER PHYSICS
of these entities in the dense medium above the substrate is too short;
the same is true of photoemission spectroscopies because the mean
free path of the photoemitted electron is too short for the electron to
emerge from the dense medium above the solid. Thus, many of the
methods discussed above cannot be applied to the systems discussed in
the opening paragraph of the present section.
However, new methods may be applied to these systems. If the
dense medium is transparent, then any photon spectroscopy may be
used to explore the interface, provided the signal from the species of
interest (at the interface) may be detected, in the presence of the
possibly large background from the dense medium. If one seeks a
species not present in the liquid, then the feature of interest may lie in
a frequency domain removed from that dominated by the dense
medium. In these situations, techniques such as infrared or Raman
spectroscopy may be employed.
If the solid substrate is transparent, then the interface may be probed
by bringing the probe beam into the interface through the substrate. If
it strikes the interface at an angle of incidence greater than that
required for total internal reflection, the optical wave field is evanes-
cent in the liquid, so the backscattered or reflected radiation provides
information only on the near vicinity of the interface. In Raman or
Brillouin studies with visible radiation as a probe, one may examine the
o
first 200 A of the liquid by this means. Signals may be enhanced by
either fabricating the substrate into a waveguide, and employing an
integrated-optics geometry, or by overcoating the waveguide by an
evaporated film and coupling the incident and scattered photons to
surface polaritons in the film. Some early experiments using these
techniques show them to be promising for future surface studies.
In an earlier section we mentioned that, under certain circum-
stances, the Raman signal from adsorbed molecules may be enhanced
enormously over that appropriate to the gas phase. However, since
surface roughness combined with the presence of long-lived surface
resonances is necessary to realize these large signals, the effect is of
limited utility for the study of single crystals in high vacuum. Fortu-
nately, just these conditions are realized in electrolytic cell environ-
ments, where, in fact, this remarkable phenomenon was discovered.
The spectra so obtained are impressive: the Raman signals from the
adsorbed molecules are so strong that they are comparable with those
produced by the solution itself. Since roughness is present on electro-
lytic cell electrodes as a consequence of cycling the applied voltage,
Raman scattering will serve as a powerful probe of the interface in
these systems.
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S UREA CES AND INTERFA CES 159
Practical catalysts employ the active material in the form of small
particles suspended on a substrate. These high surface-to-volume
systems may be probed with infrared spectroscopy; indeed, the earliest
infrared spectra of adsorbed species were obtained on such systems. A
new method of vibrational spectroscopy applicable to them has been
developed in the last decade. This is inelastic-electron-tunneling spec-
troscopy. It has been known for many years that if an electron tunnels
from one metal to another, through an insulating barrier between them
(the insulating barrier is commonly the oxide of one of the constitu-
ents), then features in the l-V curves of the structure are produced at
voltages that correspond to the energies of vibrational modes of
molecular species trapped in the oxide. Analysis of these features
allows one to deduce the nature of the molecular entities trapped in the
oxide. The method has now been applied to systems that mimic
practical catalysts, to obtain the vibrational spectra of the adsorbed
species. The resolution of the method is high, if the data are taken at
low temperatures (4 K is sufficient).
Thus, while many of the experimental methods of surface physics
rely on the use of particle beams with constituents that have a short
mean free path in matter (this is why such beams are useful for probing
the surface, of course), so that they are not applicable to the study of
the interface between a solid and any dense medium, new spectroscop-
ies are being developed that are directly applicable. The latter will
surely continue to evolve in the coming years and will provide an
important supplement to more traditional methods.
THEORY
Advances in experimental techniques for the study of structural and
dynamical properties of crystal surfaces, particularly through the use
of external probes such as electrons and rare-gas atoms, require
parallel theoretical efforts for their interpretation owing to the strong
interaction of these probes with the system being studied. The exper-
imental advances of the past decade described in the preceding four
sections have been accompanied by significant theoretical achieve-
ments.
The theory of atom/surface scattering has made significant advances
in recent years. It is now possible to calculate the intensities of the
diffracted beams fairly accurately, given the potential. Calculations
based on model potentials can be brought into impressive agreement
with data, and a detailed understanding of the physical origin of the
scattering resonances often observed has emerged.
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160 A DECADE OF CONDENSED-MATTER PHYSICS
A fundamental problem in the interpretation of atom/surface scat-
tering data for the determination of surface structures is to relate the
interaction potential to the actual nuclear positions in the crystal. It has
recently been shown that to a good approximation the repulsive part of
this potential is proportional to the electron charge density in the
surface layers of the crystal and is consequently short ranged, while the
attractive part is of the van der Waals type and is long ranged. Ab initio
calculations now can generate electron-charge-density contours 3-4 A
outside the surface, which is as close as a helium atom incident on a
crystal approaches its surface.
During the past decade, there has been a major advance in our
understanding of the electronic structure of surfaces. From early
theoretical studies, and simple pictures based on chemical intuition, it
was clear that under a variety of circumstances one should find
two-dimensional bands of electronic states localized on the surface.
The development of the technique of angle-resolved photoemission has
allowed the direct study of the surface state bands on a wide variety of
clean and adsorbate-covered surfaces. During this period, largely
through application of the density functional formalism, theorists have
carried out self-consistent studies of the electronic structure of sur-
faces. Agreement between theory and experiment can allow one to
draw firm conclusions about the structure of the clean surface or the
bonding sites of adsorbates.
In the largest number of such theoretical calculations a structure for
the surface is assumed, and a self-consistent calculation is then carried
out of the one-electron energy states, with the nuclei held fixed. The
results are then placed alongside the data, and, if necessary, the
calculations are repeated for several different surface geometries until
a match between theory and experiment is achieved. In a major
development theorists are now actively engaged in calculations of the
total energy of the surface structure, within a framework that allows
the nuclear positions to be varied. Then one may seek the configuration
of lowest energy to predict the surface structure of a given material.
OPPORTUNITIES
The achievements of the past decade provide some indications of the
areas in which research in surface physics will be carried out in the
next decade.
Surface Brillouin spectroscopy, the emergence of Raman spectros-
copy as a surface-sensitive probe, and the use of field enhancement in
a variety of optical interactions near surfaces and interfaces constitute
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SURFACES AND INTERFACES 161
a new field of experimental endeavor with substantial promise. A
factor-of-2 improvement in the spatial resolution of the scanning
vacuum tunneling microscope will be a major advance; this device will
offer surface physics a new probe that will greatly expand our
understanding of a variety of features of surface geometry.
The technique of second-harmonic generation of laser light can be
used for in situ measurements at interfaces between two condensed
media with a picosecond time resolution. This opens up many inter-
esting and exciting possibilities for surface studies in, for example,
high-pressure catalysis, electrochemistry, photolithography, and even
biophysics.
In spin-dependent scattering of electrons from a surface we have a
new surface probe that for the first time can be used to probe
magnetism in the outermost atomic layers of crystals. For example,
antiferromagnets should be readily studied by this technique, since
new Bragg beams will appear if the surface orders in such a manner
that the appropriate two-dimensional unit cell increases in size. One
may also envision inelastic scattering of spin-polarized beams where a
spin wave rather than a surface phonon is responsible for the loss.
Since we have no information on the behavior of surface spin-
correlation functions in the near vicinity of a bulk phase transition,
there would be great interest in the study of the diffuse background to
such scattering produced by spin fluctuations, particularly near a
magnetic phase transition.
However, there has been little theoretical attention paid to calcula-
tions of the magnitude and the energy and angular dependences of
cross sections associated with spin-dependent electron scattering.
Such analyses should prove helpful, by elucidating optimum scattering
geometries.
There are other questions that need to be addressed by theorists.
LEED data show that on most faces of metal single crystals there is a
contraction in the spacing between the first and second layer, with the
greatest contraction occurring for the more open faces. These results
are in disagreement with the predictions of simple pair-potential
models of the crystal. Sophisticated theory is required to provide a
framework for interpreting the data. Thus, as our ability to carry
through ab initio calculations of surface structure improves, the
information will be of direct value to LEED theorists and others
engaged in other studies of electron spectroscopy of the surface region.
The implications are exciting; LEED theorists will have in hand clear
theoretical guidance when an obvious choice of structure fails to fit the
data. The effort will lead to more reliable potentials for integration into
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162 A DECADE OF CONDENSED-MATTER PHYSICS
a variety of analyses of the interaction of electron probes with the
surface. It will also be possible to calculate, in an ab initio fashion, the
force constants that enter models of surface lattice dynamics.
A complete understanding of the photoemission process requires
knowledge not only of the electron energy states, their wave functions,
and the sensitivity of both to surface structure, but it also requires
knowledge of the electromagnetic field of the incoming photon in the
near vicinity of the surface. This is an area where further theoretical
understanding is both required and will prove fundamental not only to
photoemission spectroscopy but to other surface spectroscopies ad-
dressed in this report.
In a great deal of the theoretical work on the interaction between an
atom and a crystalline substrate the latter is treated as if it is a perfectly
rigid structure, whose only role is to provide an effective potential that
influences the motion of the atom. In fact, if an atom is placed in an
adsorption site, there is a distortion of the lattice in its near vicinity.
Also, if the atom is adsorbed on a particular site and it hops to a
neighboring site, there will be a local distortion of the lattice that will
be dragged along with it, during diffusion on the surface. As an atom
approaches a crystal, to resect off it in a scattering experiment, there
will be a local distortion in the near vicinity of the impact site. As the
atom recoils, a substantial fraction of its energy may be transferred to
the lattice. This whole sequence of phenomena requires for its eluci-
dation a description of the interaction of the atom with the vibrational
quanta of the substrate (phonons) and a theory that provides a valid
description of the consequences of this coupling. Our ability to
describe atom/phonon interactions, and to exploit their consequences,
is at a primitive stage at present, yet these couplings may play a crucial
role in many aspects of the atom/surface interaction. Computer simu-
lations may prove useful here.
So far, we have discussed only the scattering of atoms off the surface
or their motion on it, under circumstances where the electronic
configuration of the atom remains unchanged during the interaction
process. For rare-gas atoms interacting with the surface, this picture is
surely sufficient in most circumstances. However, when atoms (or
ions) with lower ionization potentials or electron affinities strike the
surface, it is possible for an electron to be transferred from the atom to
the surface or for the incoming particle to pick up an electron. A
rigorous description of these processes poses a real challenge. At
present, classical trajectory analyses form the basis for most theories,
but a fully quantum-mechanical description of the atom motion may be
required.
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S UREA CES AND INTERFA CES 1 63
Our discussion of the last two topics has focused primarily on the
need for further development of the theory. The absence of predictive
theories limits our ability to appreciate the full significance of existing
data, and if definitive developments occur in the theory, surely the
direction of experimental research will be affected positively. Another
area where the absence of theory limits our ability to appreciate the full
significance of data is electron- or photon-stimulated Resorption, in
which adsorbed atoms are detached from a surface on excitation by
incident electrons or photons.
Our knowledge of the origin and even the magnitude of lateral
interactions, particularly in chemisorbed systems, is sketchy. Since
these interactions play a key role in stabilizing the various surface
phases encountered in chemisorbed systems, and control the degree of
short-range order present in a disordered overlayer, a more complete
understanding of the underlying physics that controls their strength and
magnitude is important to have.
The study of the interaction of small molecules with surfaces, with
emphasis on the interchange of vibrational or rotational energy, is
expected to be a lively and active area in the coming years. As our
understanding of the interaction of atoms with surfaces becomes
increasingly quantitative, we acquire a base upon which a clear
understanding of molecule/surface interactions may be based.
As one moves from small, simple diatomic molecules to more
complex entities such as hydrocarbons, we approach issues of direct
interest to surface chemists. There is at present a rather limited amount
of structural data on the adsorption geometry of hydrocarbons and
hydrocarbon fragments. The complexity and variety of adsorption
geometries possible for these systems renders a full quantitative
interpretation of the data difficult, but we are seeing the beginnings of
active research in the area, with attention to quantitative results.
Several laboratories are actively exploring the use of time-resolved
methods to probe the kinetics of molecule/surface interactions in real
time. Such data could lead to a qualitative expansion in our under-
standing of the kinetics involved, by direct observation rather than
indirect inference. It is possible to envisage the construction of
apparatus that can resolve surface kinetics at the submillisecond level,
with microsecond resolution as a lower bound owing to limitations on
one's ability to chop a molecular beam. Activated rate processes are
easily slowed down by cooling the sample, so millisecond-resolution
experiments will suffice to provide a major step forward in our
understanding of surface kinetics. This is an area, virtually unexplored
at present, that should prove exciting in the coming years.
Representative terms from entire chapter:
dense medium
