Nuclear Ins~men~
and Methods in Physics ResearchB 94 (1994) 353-358
Beam tnteraetions
with Metertats& Atoms
Book review
This section of NIM B will bring reviews of books relevant for the regular readership of the journal. Books for review
should be sent to one of the Editors.
~~~E~~
PROCESSES IN SPUTTERING OF
ATOMS AND MO~CUL~
(SPUT 92)
edited by P. Sigmund. A Symposium on the Occasion of
the 250th Anniversary of the Royal Danish Academy of
Sciences and Letters. Invited Reviews K. Dan. Vidensk,
Selsk. Mat. Fys. Med. 43 (Munksgaard, Copenhagen, 1993)
pp. 675, hardcover, DKK 840, excl. VAT (approx. US$
130). ISBN 87-7304-249-S.
Level: graduate, researcher
Reviewer: R.A. Baragiola, Charlottesville
This book is a collection of chapters with review
articles written by several authors who participated in a
symposium on sputtering which took place in Copenhagen
in the summer of 1992. The chapters were written after the
conference and benefited from multiple refereeing and
interactions with the ediior. The primary aim was to
present recent (last _ 10 years) developments in the field
of sputtering, written by many of the top researchers in the
field. The book may be considered as a supplement to the
series by Behrisch et al. [l], the essential reference in the
field of sputtering.
A number of contributed papers from the same symposium have been published in a Topical Issue of MM B
(82 (2) (199311, after a similar editorial process, where a
number of new results have been reported with full technical details. There were no length limits and the papers
were submitted up till three months after the symposium,
Hence those are not conference proceedings in the usual
sense.
Many of the fundamental concepts for sputtering have
been known for decades now. Basic mechanisms are divided into elastic sputtering, where the energy and momentum required for particle ejection is provided by (mainly)
elastic collisions, and electronic sputtering, where energy
is primarily transferred to the electronic system and then
later coupled to atomic motion. Over the years, there has
been a marked, but not complete, segregation of researchers along these two lines, which has been reflected
in international conferences. Fortunately, the tendency is
now to bridge this gap of different points of view, as
exemplified by this book. Another trend is the increasing
use of steadily improving imputations
tools which have
been applied successfully to inspect sputtering mechanisms
which are continuously being proposed. Most major mechanisms are well discussed throughout the book, with the
exception of the so-called “thermal-spike”, where additional ejection is thought to occur by thermal processes
initiated by dense collision cascades. Although discussed
in passing in many chapters, there is no detailed critical
evaluation of the limits of the use of effective temperatures
to predict properties based on the~odynamic models.
This heavy volume gives a good cross section of the
current state of the art in both experimental and theoretical
treatments, ranging from traditional topics to modem developments, both of fundamental physics and applications.
Most current topics are included, with the exception of
~conda~ ion emission which, however, is pe~~ic~y
covered in books and conference proceedings [2,3].
The collection of specialized chapters on different topics has more coherence than usual, thanks to a large
involvement by the editor. However, this is far from being
a tutorial textbook. It will most likely become an essential
reference to practically anyone working in the fields of
particle interactions with solids.
Given the quality of the material presented, it is regrettable that there are several sho~~mings in the publishing
aspects. For instance, there are no useful page headers, so
it is very hard to locate where the references start for a
given chapter, or to locate an articie by just flipping pages.
Figures are many times shown several pages before they
are first mentioned in the text and, in other places, shown
several pages after they are introduced. A major shortcoming is the absence of a topic index, which will limit the
usefulness of this volume.
The book starts with a clear introduction by Sigmund,
meant to guide non experts. He presents the main experimental observables and representative models. Practitioners of molecular dynamics (MD) simulations will find the
model for the low-yield regime in sputtering to be too
simplified, since they are accustomed to looking at all
Old-583X/94/$07.~
6 1994 Elsevier Science B.V. All rights reserved
SSZIZ0168-583X(94)00291-6
354
RA. 3~r~giola/~uci.
Ins&. and Meih. in Phys. Res. B 94 (1994) 353-358
moving atoms, including the vast majority which cannot
escape from the solid. This distinction of ways to see the
collision cascade is relevant since it has been central to
many debates among theorists.
The first chapters of the book deal with computer
simulations. Their development has progressed to the point
(or even beyond) where one should start including other
aspects of reality like the presence of vacancies and interstitials, disl~tions,
steps at smfaces, etc. Theoretical
samples are usually too perfect.
Robimm’s excellent chapter updates a popular review
by Andersen [It] and includes the newer, more elaborate
many body potentials. The approach is traditional, emphasizing interatomic potentials. Coverage is very good, but
the reader will be left wondering why insulators are not
treated. The main difficulty with theory, at present, seems
to be the treatment of the surface and inelastic energy loss
processes involving the projectile and the recoiling target
atoms. For dilute cascades, a standard approximation in
non-MD models is that the surface barrier is planar. This
barrier introduces a peak in the energy distribution of
sputtered atoms which, in the often used Thompson model,
is located at half the average surface binding energy
appropriate for sputte~g. For dense cascades, several
neighing
atoms may be sputtered at the same time, so
the planar barrier should not be a good a~roximation.
Nevertheless, Thompson’s formula continues to be used to
make unwarranted inferences about “thermal” components of sputtered flux in attempts to prove the existence
of “thermal” spikes or to derive an apparent lower effective binding energy for the sputtering of condensed molecular solids.
Probably a good test of models would be provided by
testing angular and energy distributions of atoms sputtered
at grazing angles to the surface. This condition will maximize differences between planar and spherical barrier models and the effect of surface defects. Recent developments
in preparation of atomically flat surfaces should allow
meaningful experiments to be done at low ion fluence,
before the development of any significant bombardment
induced topography. Robinson reminds us that the old
mystery of the development of topography is still not
understood.
The discussion of the inelastic effects is somewhat
incomplete since it does not consider what is known from
the field of single particle, gas-phase collisions. Models for
soiids are very simplified; they often use electron gas
theories and do not take into account orbital structure,
level matching, etc. Some models try to inchtde non local
effects but only in a heuristic or empirical way, mainly
assuming that energy losses in atoms can be taken to
depend on the local electron density. The reading of this
chapter could be complemented with more specific discussions on inelastic collisions in solids [S] and with papers
dealing with inelastic effects in ion scattering spectroscopy
[3]. Robinson stresses that electron-phonon coupling in the
collision cascade is currently one of the most difficult and
obscure problems in particle-solid interactions.
Niemine~‘s chapter is a good description of MD, though
somewhat idealized. For instance, even though many aspects of the sputtering process can be incorporated in an
MD code, one cannot expect to approach an exact description. Readers may disagree with the assertion that one can
evaluate exactly the screening function in the high energy
region, since in reality, practically all calctdations are for
ground state interactions. The neglect of excited states,
which may be ~nimpo~ant in traditional solid state applications, may not be warranted in the case of collisions
involving energetic recoils. Commercially supported quantum chemistry programs are not meant to describe close
collisions, where a large number of excited configurations
are needed to describe approximately the transient quasimolecule. The readers should be prepared to soften some
strong statements in an otherwise well written review. For
instance, I would argue that it cannot be shown “rigorously” that the stopping power of a slow ion traversing an
electron-gas-like metal is proportional to ion velocity.
Also, I find too strong the assertion that “electronic friction can (and should) be implemented by simply adding a
vel~ity-de~ndent
term to the equations of motion”.
The chapter by Ur~a~~~~and Hufer discusses the still
myste~o~ topic of sputtering of moiecules and clusters,
for the case of electrically conducting solids. The emission
of clusters is one of the research frontiers in sputtering. It
provides a strong challenge to computer simulations, since
cluster emission is a rare event and since the description of
the process requires an accurate inclusion of binding forces
and different ways of distribut~g internal energy in the
cluster. The authors discuss the differences in the abundance distribution of ejected neutral vs. ionized clusters.
For charged clusters, the ~temation of this distribution
with cluster size, is connected with that of the behavior of
the ionization energy. However, correlation does not imply
connection in the sense of a cause-effect relationship. The
idea that clusters are formed as neutrals and then ionize
when leaving the surface is at odds with current understanding of other aspects of charge transfer in ion-surface
interactions.
The traditional view that cluster emission is related to
the statistics of the sputtering event is reviewed. The
authors state that statistical fluctuations must have a major
effect in cluster abundance and that one might indeed
expect that the cluster size distribution reflects the fluctuations of the individual collision cascades. They even suggest that measurements of the sputte~ng yields of
monomers and dimers at low energies may give direct
~fo~ation
of the statistics of the sputtering yield. This
idea, however, disregards the effect of the finite size of
binding forces. One is not dealing with the simultaneous
emission of it individual atoms but with a cluster of n
atoms. The emission of a large cluster not only requires the
fluctuation in the sense of a large energy deposited near
R.A. Rarag~Ia/~~ei. In&r. and Meth, in Phys. Res. B 94 (2994) 353-358
the surface, but also that the energy is distributed in such a
way that the atoms are bound in a cluster. Some of these
issues are discussed later in the book in chapters dealing
with the ejection of large biomolecules from solids.
A point that is not discussed is the tiuence of damage
and induced to~~aphy in the emission of clusters. One
may imagine that cluster emission may be favored in cases
where many bonds are already absent at the time of the
sputtering event.
In his chapter, Andersets deals with the often controversial topic of non-linear sputtering, where yields are
larger than expected from linear theory when the density
of energy deposition is high (like for energetic heavy ion
and cluster impact). The discussion is careful, especially in
the avoidance of the term thermal to refer to spikes.
Quantification of non-linear effects are based on comparisons with Sigmund’s linear theory. This is less satisfying
thau comparison with Monte Carlo codes, since deviations
from Sigmund’s theory may be given by a number of
non-essential approximations in the treatment of surface
effects, interatomic potentials, and electronic processes.
An important fact, mentioned in passing, is that the enhanced sputtering yields from high density cascades camrot
be predicted solely on the base of a high yield since, for
instance, oblique incidence gives larger yields but not
higher enhancement factors. In fact, enhancement is more
likely related to the microscopic frwc of sputtered atoms
but this would require a discussion of the time evolution of
the collision cascade, which is absent in analytical theories
and Monte Carlo simulations. This chapter also includes
some early work on crater formation but these resubs are
not easy to interpret, since craters include the effect of
lattice relaxation after the impact. The electron micrographs shown will not be understood by the majority of the
readers. The work discussed is related to cratering induced
by large molecules and micrometeorites on surfaces [6,7].
In the latter case, the emission of electrons, ions and
photons at velocities less than lo5 cm/s are a more clear
indication of non-linear effects.
Andersen’s review also mentions the “cluster fusion”
experiments where, in an aftermath of “cold fusion”
reports, it was claimed that high temperatures produced in
impact of heavy water clusters with surfaces resulted in
anomalons nuclear fusion yields. These experiments were
received with almost universal skepticism and were later
withdrawn when the original criticism of light ion contamination was proven right.
An often used “proof” for thermal spikes is the displacement to low energies of the peak in the energy
distribution of sputtered atoms, as mentioned above. Some
authors have fit a “thermal” component to the sputtered
energy spectra. This appears twenty,
since the collapse of the surface barrier brought by the high density of
moving recoils will enhance the escape probability of the
recoils and thus affect the low energy part of the energy
spectrum of sputtered atoms. It also seems strange that one
355
would assume that a bimodal energy distribution would
result, rather than a graded epsilon.
Andersen suggests that cluster emission may itself be
connected to non-linear effects resulting from fluctuations
in energy deposition and speculates that enhanced sputtering yields due to non-linear effects at high projectile
energy may turn into depressed sputtering yields at impact
energies below a few keV.
The contribution by Ens discusses the young and active
field of desorption of organic molecules. U~o~ately
there is no connection with the chapters by Andersen and
by Urbassek and Hofer. After reading the chapter one is
still left with the question: how are these huge molecules
ejected intact at t&se low ~mb~ding
energies? Many
problems are already not understood in the gas phase; an
example is excitation to predissociation states, responsible
for much of the background in the mass spectra. An
obstacle iu the understanding is the fact that only ionized
molecules are observed, since the dependence of io~tion
probabilities on experimental .parameters have not been
characterized. Intriguing are the observations of signal
enhancement in liquids and the relative insensitivity of
mass spectra to impact conditions.
Trong and Bedrossian give a progress report of research on ion impact craters by scamting tunneling microscopy. The connection with real time sputtering experiment has not been made yet, and many of the structures
observed are possibly created long after impact, when
vacancies diffuse to the surface and annihilate there. In
fact, sputtering yields calculated from the number of missing atoms in the craters typically exceed the macroscopic
sputtering yield, especially at low impact energies. Moreover, more craters are often observed than the number of
ion impacts. My impression is that this technique will
prove more useful to study the evolution of surfaces tier
impact than to give complementary imormation of the
dynamics of the impact event. The new scanning probe
instruments are likely the most adequate tools to revive the
field of ~rnb~~ent
induced topography and explain
some of its mysteries.
Winogrud discusses recent experiments on single crystals, made possible by the application of sensitive laser
techniques. These studies are very difficult but can be a
good test of MD ~c~atio~,
once surface defects are well
characterized and entered into the simulations. Notable are
the surprising results of Burnett and co-workers of a
two-fold decrease in the sputtering yield of single crystal
ruthenium after only a monolayer or so has been removed
by sputtering. Since theoretical results are often compared
with experiments under higher dose conditions, the ruthenium results suggest that theories may be off by a factor of
Z! More experiments are needed to confirm these results
with other techniques, which will also serve to validate the
laser post-ionization technique.
The comprehensive chapter on alloy and isotope sputtering by Sterna
and Lam touches some of the most
356
RA. Baragida /Nucl. lnstr. and Me& in Phys.Res. 3 94 (1994) 353-358
difficult aspects of sputtering, relevant for many applications: preferential sputtering, recoil mixing and recoil implantation. The difficulty in the description of these processes is that the motion of very low energy recoils is very
implant when discussing differences of atomic composition of the solid between the oute~ost surface layer and
the second layer. This occurs when describing recoil implantation, when discussing the depth of origin of the
sputtered atoms, and when analyzing differences in composition measured by ARS (sensitive to a few atomic
layers) vs. ISS (sensitive mostly just to the surface). At
this microscopic level of description, theoretical estimates
will depend very critically on many different parameters
which are outside the range of analytical transport theories
and Monte Carlo simulations. This sensitivity is also seen
in the difficulty of precisely specifying the experimental
conditions for testing various physical models.
The authors succeed in providing a rather comprehensive review, which will most likely be an inevitable reference in future work on this subject. Fortunately for the
readers, the authors did not suppress their critical views
nor were afraid to risk opinions on controversial topics,
like isotope effects. Also useful is the work by the authors
in reconciling different views of previous theoretical work
by many authors and defining a list of standing problems.
Readers should be aware that the many concepts treated in
this chapter are insufficient when discussing insulators,
where long range unbalanced forces result from electrical
charging effects that accompany irradiation. More discussions of insulators are given in later chapters by Johnson
and Schou, by Szymonsky, and by Haglund and Keliy.
Reimam complements the chapter by Ens, discussing
mechanisms for fast ion induced desorption of large biomolecules. The fact that these very large bio-molecules
(comprising several thousand atoms) are ejected intact
from surfaces when impacted by highly ionizing particles
is surprising at first sight. ft requires intermolecular bonds
that are much weaker than intramolecular bonds, and a
momentum transfer to the desorbed molecule occurring far
from the “hot” region of large energy deposition set up by
the projectile. That is, desorption of intact molecules results from either distant momentum transfer interactions or
from the transport of coherent excitations far from the
ionization tracks, where the molecules will most likely be
fragmented. The author discusses different models, like
collisional, thermal spike, and shock waves, which appear
to describe partial aspects of the ejection process. The
di~culty is that many models describe some general trends
of the measured yield of sputtered molecular ions, while
most of the sputtered flux is neutral. Reimann favors the
pressure-pulse mechanism by Johnson et al., where wrrelated momentum transfer from the projectile track may
push the biomolecule away from the surface. This model
has been successful in explaining both the stopping power
dependence of the yield and angular distributions in the
only experiment that has been made on bulk desorption of
large neutral molecules. The pressure pulse mechanism
requires coherence in the sum of momentum transfers,
which means that the electronic energy needs to be transferred into atomic motion in times of the order of or faster
than lattice response times. This will not occur in substances like the rare gas solids, where the conversion of
electronic energy into nuclear motion spreads over the
long radiative lifetimes of the intermediate exciton states.
However, a coherent expansion around an ionized track
will occur in any insulator during the time needed for
electron-ion recombination.
This chapter is complemented by an experimental one
on desotption by fast particles by Hiikansson. Again,
except for one case, the data is for desorbed ions, which
are more easily detected than atoms. Experiments are
discussed in terms of track effects, including the surprising
generation of C, molecules from a polymer containing a
large proportion of H and F atoms, a phenomenon which is
far from understood. In the case of desorption of Hf from
surface species it is likely that repulsive states formed by
electron capture by the projectile also play an important
role in desorption, as judged from the dependence of
desorption yields on the charge of the projectile. The case
of energy and angular distribution of the low energy ions
is consistent with the pressure pulse model, although it is
not clear what role is played by electrostatic effects or
what is the dependence of the ionization probability on
experimental conditions.
The extensive chapter by Johnson and Schou covers
both theory and experiments on desorption of condensed
gas solids and inorganic insulators. The chapter excludes
alkali-halides, which is treated separately by Szymonsky,
this is unfortunate because a coherent, unified, picture is
long over due. Electronic sputtering is a particularly important topic because it provides one of the few views of
non-radiative relaxation processes in solids. Unlike sputtering by elastic collisions, electronic sputtering is extremely
dependent on the properties of the target. Sputtering yields
have been found to have a linear to a cubic dependence on
the inelastic energy deposited near the surface. This depends on the density of energy deposition and on target
properties like binding energies, electron and hole mobilities, whether the sample is atomic or molecular, the presence of impurities, etc. The picture is further complicated
by the fact that the proportionality factor with a certain
power of the inelastic energy deposition depends on
whether one is in the limit of low or high velocities. This
causes a two-branch or loop behavior of the dependence of
the yields on stopping power. The authors give different
possible explanation for this behavior, which is not well
understood: 1) at high velocities, comparisons should be
made with the stopping power that correspond to the initial
charge of the projectile, rather than with tabulated values
that correspond to equilibrium charge of the projectile. 2)
the effect of a velocity dependent partition of energy loss
among excitation channels with different probabilities of
RA. Sara&da / Nucf. instr. and Meth. in Phys.Res. 3 94 (19941353-358
ending in atomic motion, 3) decrease in the ion~ation
track density at high velocities due to a larger extent of the
ionization track, 4) decrease in the energy deposition near
the surface at high velocities due to transport of energy
away by fast electrons. Item 3) should include the
velocity-dependent role of inner-shell excitations, which
are known to be efficient in other desorption processes,
through hole localization.
Other processes of, desorption induced by electronic
transitions can be found in the literature under the description of electron stimulated desorption [8] and photochemistry or photodesorption [9], and are usually treated separately in the literature and at scientific meetings [lo].
Studies of photon and electron stimulated desorption often
deal with single adsorbed layers and generally analyze
only the desorbed ions, which are easier to detect, but
which comprise only a very small fraction of the desorbed
flux. A current topic connected to this chapter is desorption of surface molecules by dissociative attachment of
photoexcited electrons from the substrate, transmitted
through thin adsorbed layers. Such a mechanism could
explain the enhancement of the sputtering yield of condensed hydrogen for thin fdms.
The old topic of sputtering of alkali halides has been
shaken lately by detailed experiments and new theoretical
concepts. The new notation
on preferential sputte~g
of the alkali and halogen components, obtained under ion,
electron and photon impact is treated consistently in the
chapter by Szymonsky, who also refers to interesting recent
results of sputtering of free alkali halide clusters. The
preferential sputtering depends on temperature, with a
collisional component for halogen ejection superimposed
on thermal sublimation components. The hyperthermal
component, which is the most interesting, is only observed
for some of the alkali halides.
Traditional mechanisms invoked for the electronic sputtering of hyperthermal halogen atoms are not too different
from those in the rare gas solids. Energy deposited in
electron-hole pairs is localized in trapped holes and excitons. The subsequent decay processes involve non-radiative transitions which result in energetic recoils in the bulk.
The atomic motion then propagates to the surface, possibly
causing sputtering. Different from the case of rare gas
solids, a replacement collision sequence is invoked for
alkali halides, occurring along rows of halogen atoms. A
recently proposed “surface” mechanism involves transport of electronic energy by “hot” holes from the bulk to
the surface. The holes, being energetic, can localiie at an
unstable, neutralized, surface halogen. This repulsive congyration can then relax by ejecting the halogen atom with
a hy~~e~~
energy. This mechanism is somewhat related to the mechanism of cavity ejection in rare gas solids,
where hot excitons diffusing from the bulk localize in a
repulsive surface configuration.
Haglund and Kelly discuss ablation of insulators induced by pulsed laser beams. Ablation is defined as a
357
condition where sputtering rates increase faster than linearly with laser intensity. As such, this includes photodesorption by two-photon absorption, and the cumulative
effect of multiple photons, such as standard thermal effects, photon interactions with the previously excited solid
and desorbed species, etc. In ablation, electron-hole pairs
are originally excited by single- or multiple-photons (depending on the density of bandgap states such as surface
and impurity states). The excitation then transforms into
heat with an efficiency which depends on the strength of
the electron-lattice coupling, which is very sensitive to the
type of bonding. At large photon fluxes, the response of
the material to an incoming photon is affected by alteration
produced by previous photons in the pulse. Heat causes
material to evaporate in a plume which can also absorb
energy from the laser beam leading to photoionization in
the gas phase and collision chemistry in the plume. At high
photon fluxes multi-photon excitations to repulsive molecular states are possible, so it is not surprising that hyperthermal atoms are observed in the plume. This topic is
developing rapidly due to manufacturing, engineering and
medical applications.
Bridging between the chapter of Haghmd and Kelly
and those on ion impact desorption of biomolecules is that
by Karas on matrix-assisted laser desorption. Thii is a
very recent technique, which has found widespread use in
analytical mass s~c~orne~,
because of the ability to
desorb even larger bio-molecular ions than can be desorbed using fast incident ions. In this method, the laser
energy is preferentially absorbed by a matrix on which the
biomolecule of interest is embedded. This energy absorption, which results from either a resonant or a multi-photon
process, leads to an explosive evaporation which results in
desorption. The mechanism may be similar to the pressure
pulse model of Johnson et al., although there are several
experimental observations which apparently do not fit this
picture,
Taglauer touches on some recent applications of sputtering to surface science. They include the so-called chemical sputtering which happens in low energy discharges in
reactive gases, which are seeing widespread technological
use. This chapter is somewhat out of balance with the rest
of the book due to its brevity. It can be complemented by
other recent books [ll-131 and reviews [14].
The last chapter is by Tombrdlo who describes selected cases where sputtering is important in planetary
science. He focuses on the effect of irradiation by ions
from the solar wind. Unlike controlled experiments in the
laboratory, here sputtering is one of many possible effects
which include meteoritic impact, phot~eso~tion
and de~m~sition, and volcanic processes. Furthermore, the dose
rates in these environments are usually many orders of
magnitude smaller than those that can produce meaningful
data in the lifetime of a human experimenter. Interesting
topics included are the survival of cosmic grams, how
grain-grain collisions can be simulated by molecular dy-
358
RA. Baragiola/Nucl.
Instr. andhfeth.
namics, and the effect of irradiation on isotopic ratios
which are used to obtain information on astronomical
history. A particularly far reaching application of sputtering is the mapping of surfaces exposed to the solar wind
by mass spectrometry analysis of ejected ions and neutrals.
This chapter can be complemented by a recent monograph
on the topic [15].
In summary, this is an excellent book which captures
the cardinal questions that currently preoccupy scientists in
the field. It should be in the bookshelf of any researcher
working on the interaction of particles and laser beams
with solids.
References
[l] R. Behrisch and K. Wittmaack (eds.) Sputtering by Particle
Bombardment
III, Characteristics
of Sputtered Particles,
Technical Applications (Springer, Berlin, 1991) is the latest
volume in the series.
[2] M.L. Yu, in Ref. [I].
[3] See the proceedings of the latest International Workshop on
Inelastic Ion-Surface
Collisions,
M. Bemheim
and J.P.
Gauyacq (eds.), Nucl. Instr. and Meth. B 78 (1993).
in Phys. Res. B 94 (1994) 353-358
[41 H.H. Andersen, Nucl. Ins&. and Meth. B 18 (1987) 321.
[Sl See papers by F. Flores and by 2. Sroubek, in: Ionization of
Solids by Heavy Particles, ed. R.A. Baragiola (Plenum, New
York 1993).
b1 J.A.M. McDonnell (ed.), Hypervelocity Impacts in Space
(Univ. Kent, Canterbury, 1992).
171 R.A. Baragiola, Nucl. Instr. and Meth. B 78 (1993) 223.
k31R.D. Ramsier and J.T. Yates, Surf. Sci. Rep. 12 (19911 243.
Dl X.-L. Zhou, X.-Y. Zhu and J.M. White, Surf. Sci. Rep. 13
(19911 73.
[lOI A.R. Bums, E.B. Stechel and D.R. Jennison (eds.1, Desorption Induced by Electronic Transitions, DIET V (Springer,
Berlin, 1993) is the latest proceedings of these meetings.
ml D.J. O’Connor, B.A. Sexton and R.St.C. Smart (eds.), Surface Analysis Methods in Materials Science (Springer, Berlin,
1992).
WI E.S. Parilis, L.M. Kishinevsky, N.Yu. Turaev, B.E. Baklitzky, F.F. Umarov, VKh. Verleger, S.L. Nizhnaya and J.S.
Bitensky, Atomic Collisions on Solid Surfaces (North Holland, Amsterdam, 1993).
Interactions
[I31 J.W. Rabalais (ed.), Low Energy Ion-Surface
(Wiley, New York, 1994).
[14] K. Wittmaack, in Ref. [l].
[15] R.E. Johnson, Energetic Charged-Particle
Interactions with
Atmospheres and Surfaces (Springer, Berlin, 1990).