Table of contents

The Fourth International Workshop on Inelastic Ion–Surface Collisions was held at Hindsgavl Manor near Middelfart, Denmark from 21 to 24 September 1982, following previous workshops held in Murray Hill, New Jersey (1976), Hamilton, Ontario (1978) and Feldkirchen-Westerham, Bavaria (1980). Like in the previous meetings, the underlying idea was to gather a moderately small group of researchers to discuss fundamental physical and chemical problems in a number of areas that are related, but are normally represented at separate conferences focusing on different aspects.

The area of inelastic ion–surface collisions has a wide diversity of applications ranging from surface analysis by particle impact through microelectronic and controlled thermonuclear fusion devices to biomolecule identification and solar wind effects in planetary space. There are strong links to surface science and atomic collision physics and their respective applications. The present series of workshops is an attempt to focus on fundamental problems common to all these areas and thus to provide a forum for fruitful interaction.

At Middelfart, we were lucky to have an exceptional number of well-presented and stimulating summary talks covering a rather broad range of fundamental processes with the emphasis shifting back and forth between collisional and surface aspects. Moreover, there was a wealth of short contributions on current research, of which many were submitted to the present proceedings. Thanks to the speakers, an active audience, and considerate session chairmen, we had extensive and lively but friendly discussions in an always stimulating atmosphere.

This volume contains 11 of 13 invited papers and 15 of the 30 contributions presented orally at the workshop. It should, like the proceedings of the previous workshops, give a balanced survey of the current status of the field, with a slight bias toward recent developments like those in the theory of charge states of sputtered atoms, and others. All papers have undergone a normal, and occasionally extensive, refereeing procedure.

In the midst of the editing process, I received the news that one of the invited speakers, Morton Traum of Bell Laboratories, had died at age 41 on 1 December, 1982 in Stoughton, Wisconsin. Mort had delivered a superb talk on Desorption and Sputtering by Electronic Processes and had been one of the most active participants and perhaps the most broadly oriented one of the workshop. His intense curiosity and serene charm, combined with a solid background in all parts of surface science, contributed stimulating ideas to most of the topics discussed.

In preparing the workshop, I got much useful advice and constructive criticism from the members of the international committee. The assistance of the members of the local committee, Nils Andersen, Flemming Besenbacher, Jens Nørskov and Jens Onsgaard, as well as Erling Hartmann, Tove Nyberg and my wife Pia was instrumental at various stages before, during and after the workshop. Generous funding was received from the Office of Naval Research, the Danish Natural Science Research Council, NORDITA, the Nordic Accelerator Committee, the Research Foundation of Odense University and the Danish Provincial Bank. It is a pleasure to acknowledge the professional service of the Hindsgavl Conference Center and the smooth cooperation with Dr N R Nilsson, executive editor of Physica Scripta.

When an atom or ion is scattered from a metal or other surface the scattered particle may be found in a distribution of charge states. He⁺ scattering is normally assumed to be dominated by Auger neutralisation for which the simple semiclassical theory is discussed. Many charge exchange phenomena actually occur via the direct or resonant electron transfer process of which we give examples. The correct theory of this process is a quantum mechanical or amplitude one. We give a simple account of this theory and emphase the limiting formulae which can be obtained under simple physical assumptions. A detailed application to the neutral fraction of Na scattered from W(110) is given and compared with experiment. Up to now the effect of parallel velocity has not been taken into account. We show how this can be done on a jellium surface by a displacement of the Fermi sphere in momentum space. The results are applied to the H⁻ fraction in H⁺ scattering from cesiated W(110) which shows a strong parallel velocity effect.

A very simple derivation is given of the ionization probability P⁻ for negative ions sputtered off a metal surface. The result P⁻ ≈ e^{−(ϕ−A)/cv⊥} shows the experimentally observed dependences on work function ϕ, atomic affinity A, and outwards velocity v_⊥. Estimates of c show good agreement with experiment, too. The physical picture behind the process is discussed.

The shift and broadening of the valence and affinity levels of an atom interacting with a metal surface are calculated at all distances outside the surface using a linearized Fermi–Thomas approximation to calculate the Hamiltonian of the bond electron, and first order perturbation theory. The effective charges on both levels are then calculated for all distances from jellium metal at zero temperature. A semi-classical approach is then used to illustrate the relative importance of the charge state in the bulk, the surface, and near surface regions to the final charge state of hydrogen reflected from gold in the energy region 0.1–8 keV.

It is shown that in a simple quantum-mechanical model the excitation probability of neutral atoms ejected from solid state surfaces is proportional to exp (− ΔE/2γℏvc) where γ defines the spatial dependence of the inter-atomic electronic interaction, ΔE is the excitation energy, v is the normal velocity of the sputtered atom and c is an adjustable coefficient. To fit the experimental data reported by Yu et al. the coefficient c must be taken equal to ten.

In recent years there has been a rising interest in atom–metal surface interactions. Especially resonant charge exchange in atom–metal surface scattering has been the subject of many theoretical and experimental studies. Atoms which easily surface ionize like the alkalis when adsorbed at a metal surface form ionic bonds between adsorbate and substrate resulting in a surface dipole. A surface coverage of about half a monolayer then results in a dipole layer, by virtue of which the work function of the metal is decreased by a factor of three. Especially the pioneering work of Gadzuk resulted in a theoretical description and calculation of this effect from first principles. These theoretical considerations also offer the possibility of a quantitative prediction of surface ionization. Recently the possibility of surface ionization – positive as well as negative ionization – by scattering ions or neutrals from a metal surface has been explored very intensively. This large interest is mainly due to the possibilities of forming intense H⁻ and possibly Li⁻ ion beams by scattering the respective positive ions from low workfunction metal surfaces, like cesium covered tungsten. The intense negative ion beams after neutralization will be needed for heating plasmas in connection with fusion. This paper will be concerned with the theoretical and experimental progress in the field of surface ionization.

Measurements of the negative charge-state fraction of the reflected particles resulting from D₂⁺ bombardment of a thick Na target are presented. Incident energies of 6 keV and 12 keV produced particles with exit energies between 400 eV and 5000 eV. The incident angle was 0° from the surface normal and the exit angle was 85°, giving a scattering angle of 95°.

The negative charge-state fraction was 0.20 at an exit energy of 400 eV and decreased monotonically to 0.05 at an exit energy of 5000 eV. The negative charge-state fraction was also independent of the incident energy. The present results are compared to measurements made at an incident angle of 85° with the same exit angle as above, where there is a remarkable difference in the negative charge-state fractions. The differences in the results for the different incident angles are discussed in terms of a trajectory-dependent formation probability of the negative ions.

The charge exchange between He⁺ and Ni(110) and adsorbed O has been studied in the energy range of a few 100 eV in comparion with Li⁺ scattering. The Li⁺ yield as a function of the scattering azimuth can be calculated neglecting neutralization effects. He⁺ ions in contrast undergo two different neutralization mechanisms, one being a surface electronion interaction only, the other involving very likely an interaction with bulk electrons. The second mechanism causes a very high neutralization probability for ions scattered from the second or deeper layers and for those undergoing multiple scattering trajectories. The adsorbed oxygen causes additional trajectory dependent neutralization. The azimuthal dependence of the He⁺ scattering from the Ni (2 × 1)O structure is described by numerical calculations for the kinematic part and a trajectory dependent neutralization model. The experimental results can only be partly reproduced by these calculations.

Secondary photon emission of sputtered atoms is reviewed in the light of current theories. Laser fluorescence (LFS) coupled with Auger spectroscopy for the first time measures the fractions of atoms sputtered in low-lying (0–2 eV) electronic states as a function of monolayer oxygen coverages. A "causal" collision model based on TRIM code calculations successfully accounts for the observed excitation functions.

The ionization probabilities of sputtered particles depend drastically on the chemical composition of the upper layers of the target. The enhancement of the yield of negative ions by cesium flooding is correlated to the lowering of the work function. The electron affinity level controls the relative yields of elements. The effect of the velocity of the departing particle on these yields is examined.

The yield of positive ions is increased by oxygen flooding. Although matrix effects may be strong, there are simple approximations which allow to relate atomic concentrations and ionic intensities on a large class of samples: metallic alloys, silicate minerals.... An isotopic effect has been observed which has been mainly attributed to the variation of the ionization probability with the velocity of the escaping particle: the heavier the isotope, the lower its ionization yield. A bond-breaking model is presented which explains some of the features described in the paper.

The effect of Li deposition on the sputtering of Si⁺ from oxygenated Si surfaces has been studied. It is observed that at low oxygen coverages, the Si⁺ yield decreases exponentially with the Li induced decrease of the work function ϕ. With the formation of thermally grown silicon oxide on the surface, the Si⁺ yield deviates from the simple exponential dependence on ϕ. The Si⁺ yield becomes independent of ϕ for an appreciable range of ϕ in cases of heavier oxidation.

The effect of work function changes on experimental secondary ion energy spectra is discussed. In agreement with theory the measured ion intensities frequently exhibit an exponential work function dependence. However, the predicted velocity dependence is only observed at fairly high secondary ion energies. In the absence of a velocity dependence of the degree of ionization measured shifts of energy spectra reflect work function changes directly. Various instrumental problems are shown to aggravate a detailed comparison between experiment and theory. Significiant artefacts must be expected if the extraction field is of the order of or less than the lateral field induced by a work function difference between the bombarded spot and the surrounding sample surface.

The range distribution of N₂⁺ in Cu and Ag has been determined for incident ion energies of 1 to 5.0 keV. The distributions consist of a sharp peak at the surface and a pronounced shoulder deeper into the substrate. The shoulder is characteristic of the distribution expected in the substrate and its position and energy dependence agrees well with theory. The origin of the surface peak is shown to be due to radiation enhanced diffusion.

Auger transitions involving outer electrons in free atoms and in solids are compared with particular reference to L₂₃ deexcitation in elements Na to Si. Under electron or x-ray excitation the elemental solid state spectra are band-like in character with breadth twice that of the conduction valence band, and a shape determined in detail by matrix element, surface and electronic relaxation effects. This is in strong contrast to the L₂₃ VV spectrum of Cu which is quasi-atomic in character and the M₄₅ VV spectrum of various Ag alloys which show mixed atomic-band like behaviour. Under ion excitation a superposition of a solid state spectrum and an excited atom spectrum is observed. Various theories of the origin of the quasi-atomic spectrum are considered, and attempts are made to account for the energies in the atomic spectrum and the relative intensities of atomic and solid state features. Calculations of the detailed motions of atoms in the surface region following inner shell ionisation are reviewed, and the sensitivity of atomic yield to inner core lifetime and the angle of incidence of the ion beam is discussed.

Measurements of low energy electron spectra and the total electron yield γ from energetic E/M > 20 keV/U ion/solid-interaction are used to obtain information on electron production and emission processes. The data for monoatomic projectile ions can be explained in the framework of current theories. Comparing the isotachic yields γ of atomic and molecular projectile ions a strong molecular effect on the total electron emission is found. It is explained by an interference on the wake potentials induced in the solid by the two Coulomb-exploding nuclear fragments. From these data the wake wavelength or the plasma frequency of the electron in the solid can be calculated.

Electron- and Photon-Stimulated Desorption can directly or indirectly provide atomic and structural information on the chemical state of surfaces and adsorbates on surfaces. Information is contained in the variation of desorption yield with chemical state, ion energy distributions, ion angular distributions, and in desorption thresholds and spectral dependence. Current models for desorption discuss both ionic and covalent systems and provide considerable insight into the environment of desorbing species. Widely studied examples include oxygen on metals, CO on metals, hydrogen on metals and semiconductors, and cryogenic molecular solids as models for desorption. All of these systems show variations in ion yield with chemical state of the desorbing species which in many cases fit existing models and in many cases suggest new mechanisms of desorption. Ion energy and angular distributions provide signatures of the type of state yielding desorption and the local structure of the desorption site. Desorption spectra can provide electronic and structural details of the bonding site from the analysis of near edge and extended fine structure. Near edge structure can also be used as a signature by comparison to systems of known chemical environment. The above examples of the combined use of the different variations of the desorption experiment are discussed.

During the ion bombardment of Rh(001) covered with CO ion induced desorption is observed. For ion energies below 1 KeV the desorption cross sections for He⁺ and Ne⁺ are essentially equal. The secondary electron energy spectra observed under identical conditions indicate an Auger-neutralisation of the He⁺ involving the 5σ–CO bond, causing probably the enhanced desorption.

We present a theoretical model for non-cascade sputtering processes in which these processes are considered to be induced by vibrational excitation. We discuss in particular the application of this model to sputtering (both with and without charge transfer) by heavy ion bombardment of insulating targets. In addition, we show how this theory can be used to predict the translational energy distribution of the sputtered species and compare calculations with the experimental energy distribution of SF₆ molecules sputtered from frozen SF₆. The agreement between theory and experiment is reasonable.

The interaction of high energy ions in the region of electronic stopping (1 MeV u⁻¹) stimulates the desorption of massive molecular ions of biomolecules such as insulin. The experimental details of the measurements are given with some examples of application for analytical mass spectrometry. Studies on the role of the incident ion (accelerator beam experiments) are reviewed as well as the contribution of the matrix to the desorption–ionization process. How the electronic relaxation process couples to desorption–ionization is a central question in understanding the overall mechanism of the process.

At the GSI in Darmstadt/FRG thin foils of Al, Sc, Y and Ta were bombarded with ⁵⁸Ni, ¹³²Xe and ²³⁸U ions having velocities between 0.2 and 1.4 cm ns⁻¹. Secondary ions emerging from the metal surface of these foils were investigated by TOF techniques under UHV conditions. Apart from metal ions also ions of a few impurities were observed being residues of an ion-etching-cleaning procedure. Comparing ion yields as function of projectile velocity and also shapes of mass lines, the emission of metal ions is obviously induced by atomic collision cascades and that of impurity ions by processes related to electronic excitation. Therefore, ion emission related to electronic energy loss is not restricted to dielectric material as has been concluded from earlier experiments.

In this paper some recent developments in the field of atomic and molecular inelastic scattering from surfaces are highlighted. At the low energies involved (20 meV to 2 eV) the incident particles probe the topmost surface layer in a nondestructive manner. Inelastic scattering of light particles (e.g. He atoms) is dominated by single phonon excitation or annihilation and thus provides a probe of surface vibrational structure or other accessible energy loss channels. High resolution time of flight techniques coupled with supersonic beam sources provide measurements of surface phonon dispersion relations out to the zone edge. The scattering of heavier atoms and molecules is more complicated. Multiphonon processes and excitations of molecular internal states are involved. Both time of flight and state specific detection techniques must be applied to get a full picture of molecular inelastic scattering. This is illustrated by studies of rotational energy transfer in the scattering of NO from Ag(111). Coupling of translational to rotational motion is found to be a dominant energy transfer channel while vibrational excitation has a very small probability. Measured rotational state distributions exhibit several interesting features including rotational rainbows and rotational alignment. Quantum mechanical calculations on a simple model of the scattering provide a semi-quantitative description of the results and allow information on the interaction potentials to be derived.

The study of linear and circular polarisation of light induced by inelastic ion-surface collisions has revealed interesting properties of the ion-surface interaction. Application of the technique to single atomic collisions allows investigation under very clean conditions. Coherence and correlation analysis yields in favourable cases a direct visualisation of the shape and dynamics of the electron cloud of the excited atom. The principle of the technique is presented, and some recent examples are selected. The degree to which this information provides new insight into the physical processes governing the excitation is discussed.

The interaction of fast ions with thin foils and solid surfaces yields an anisotropy in the spatial distribution of the orbital angular momenta of the excited levels. The current status in investigating this phenomena and the applications in atomic spectroscopy and nuclear physics are discussed.

In detailed investigations, the scattering and the interaction mechanisms of 70 to 200 keV He⁺ ions at grazing incidence on a monocrystalline (110) NiFe surface are studied both by experiments and by computer simulations. The light intensity I and the Stokes parameters S/I, C/I, and M/I of the He⁺–468.6 nm and of the He⁰–587.6 nm radiation emitted by the scattered particles were observed as function of the angles of incidence relative to surface lattice directions and of the cleanness of the surface. The results exhibit a strong dependence of I and S/I on the surface crystallography and coverage. An interpretation of the results is proposed on the basis of two excitation mechanisms, (i) excitation by electron capture from the metal surface, (ii) excitation by collisions with adsorbed atoms, and (iii) an Auger deexcitation or resonance ionization mechanism which quenches the light emission of particles with small scattering angles.

Radiative deactivation of metastable hydrogen (H(2s)) atoms at a polycrystalline Ta-target has been studied in the energy range between 100 meV and 8 eV. One quarter of the impinging H(2s)) atoms are deactivated by Ly-α-emission. We propose a model after which 50% of the incoming H(2s) atoms are deactivated by resonance ionisation, whereas the remaining H(2s) atoms are scattered equally into the 2p and 2s final states.

The energy distribution of low energy helium ions (∼ 500 eV to 1500 eV), scattered by (111) silicon surface, has been measured in different experimental conditions. The energy at the maximum of the single scattering peak was found lower than that calculated from the binary elastic collision model. This is interpreted as the result of the electronic interaction causing an additional inelastic contribution in the energy loss.

From the angular measurements, it is concluded that this electronic process is mainly a local interaction between the ion and the target surface atom rather than a continuous process between the ion and the surface. This inelastic energy loss is found to be proportional to the primary ion energy and to depend weakly on the scattering angle. Comparison is made with theoretical models.