Substantial fractions of fast atoms or ions are converted to negative ions during grazing scattering from a clean and flat monocrystalline surface of alkali-metal halides. We interpret the experimental data by a model of local electron capture from the halogen sites of the crystal in binary-type collision events. Due to the band gap of the insulator, the probability for subsequent electron loss is low, resulting in large fractions of negative ions that survive from the collisional formation.

Oxygen atoms and ions with energies ranging from 2 to 100 keV are scattered at grazing angles of incidence from a LiF(100) surface. We analyze the charge states of the scattered beams as a function of the projectile energy. The most striking result of our study is the observation of negative ion fractions of up to about 70%. We interpret these large fractions of 0 ions to capture in binary-type collisions of fluorine 2p electrons from the target and the subsequent suppression of loss for these captured electrons due to the energy gap of the insulator. PACS numbers: 34.70.+e, 79.20.Rf To date, studies of the interaction of charged particles with insulating surfaces of ionic compounds have focused, in particular, on electron-and ion-stimulated desorption and sputtering phenomena [1 -5] and on ion-induced electron emission [6 -8]. Recently Souda et al. [9] reported on ion scattering studies from those surfaces, in which the mechanisms of neutralization and negative ion formation were investigated. They found evidence for a local nature of charge exchange in contrast to similar interactions of atoms in the vicinity of metal surfaces. The trajectories of the projectiles in these studies of large angle scattering result predominantly from single binary collisions with individual atoms of the target.We present here the results of the first studies of charge exchange phenomena arising from the scattering of fast atoms and ions from the surface of an insulator under a glancing angle of incidence. In this geometry of atom-or ion-surface collisions, the trajectories result from small angle scattering events of the projectiles with a large number of atoms in the surface plane ("surface channeling" ) and are nearly identical for all projectiles. Our work shows clearly that the use of an insulator as a target produces, under these conditions, new effects in the interaction of atoms with surfaces.A striking feature in our studies is found for the case of oxygen atoms and ions. Here we obtain under specific kinematic conditions for the projectiles fractions of negative oxygen ions of up to 70% for a beam scattered from a monocrystalline LiF(100) surface. Usually negative ion formation at the surface is viewed as due to the resonant electron tunneling from occupied states of the surface to the affinity levels of the ions [10,11]. This process is only effective when the work function of the surface is comparable to the affinity energies (typically in the eV domain).Thus for high yields of negative ions the intuitive concept is to use work functions as low as possible. Within this concept our result appears initially very surprising, since occupied electronic states of the alkali halide LiF (F 2p band) have binding energies larger than about 12 eV [12].The main subject of this paper will be a brief report on our experiments and an outline of a simple interpretation of our results. 0+ ions, and, in particular, oxygen atoms with energies ranging from 2 to 100 keV, are scattered from a clean and fiat LiF(100) surface under a glancing ...

We have measured the energy loss of slow protons scattered with energies from 300 eV to 28 keV from a clean and flat LiF(001) surface under a grazing angle of incidence. Our data reveal a threshold behavior of stopping at low projectile energies. The effect on the outgoing charge state indicates that electron capture and loss are dominant mechanisms for the stopping of slow protons interacting with a wide-band-gap insulator. Our data allow one also to deduce information on charge transfer in front of the surface of an insulator. [S0031-9007(98)07770-9] PACS numbers: 34.50.Bw, 79.20.Rf Stopping of atomic projectiles in matter is an important subject in fundamental and, in particular, applied research. An interesting problem in this field, that has been paid increasing attention to recently, is the (electronic) stopping of light ions with low velocities (y , y 0 , y 0 is the Bohr velocity) in insulators. From simple intuitive arguments one would expect that the band gap of insulators will suppress electronic excitation phenomena by the projectiles. Then stopping of slow light ions in insulators should clearly differ from stopping in metals, where excitations of conduction electrons close to the Fermi energy play a decisive role [1]. Similar as for the stopping by noble gas atoms, where substantial electronic excitation energies give rise to threshold effects for projectile stopping [2], one should observe similar effects also with insulator targets.In a recent paper Eder et al.[3] reported on the energy loss of slow protons traversing thin foils made from large-band-gap insulators. No threshold effects for projectile stopping in insulators (Al 2 O 3 , SiO 2 , or LiF) are observed down to energies of about 2 keV. Instead, the stopping power shows the linear dependence 2dE͞dx ϳ y. The authors interpret their data in terms of excitations owing to a local reduction of the band gap of the target in collisions of the projectiles with target atoms.Motivated by these studies, we investigated the stopping of slow protons by an insulator for projectile energies down to 300 eV. We observe a threshold behavior in the stopping by the wide-band-gap insulator LiF ͑E g ഠ 14 eV͒. Since defined energy losses of atomic projectiles at those low energies with solids can hardly be measured via transmission through solid matter, we have scattered the incident ions (protons) from the surface of a crystal target under a grazing angle of incidence. Under this condition the projectiles do not penetrate the solid but are reflected specularly from the surface with a distance of closest approach of typically 2-3 a.u. The overall energy loss of scattered projectiles is generally much lower than the initial energy (typically some percent). Data on projectile stopping obtained via bulk transmission or surface scattering can be compared to some extent, since (1) at low velocities small impact parameters resulting in larger angular deflections play a negligible role, and (2) the electronic structure of the LiF target is practically identical in t...

Xe^"*" ions with charge up to ^ =33 and energies 3.7^ keV are scattered under a grazing angle of incidence from a clean and flat Al(l 11) surface. Because of the image charge interaction the ions are accelerated on the incident path towards the surface plane which results in increased effective angles of incidence for the scattered projectiles. From the angular distributions for reflected neutralized projectiles we deduce the image charge interaction energies gained by the incident ions in front of the surface. Our data are in fair agreement with a q ^^^ dependence for the image energies as predicted from a simple classical overbarrier model. PACS numbers: 79.20.Nc, 34.70.+e, In recent years the study of the interactions of slow multiply charged ions with surfaces has developed into a very active field of research in particle solid interactions.Investigations of x rays and in particular electron emission have provided important data for elucidating the relevant interaction mechanisms. There is convincing evidence that the multielectron capture and loss processes in front of a metal surface result in the population of multiply and highly excited levels in the projectile and that most of the sparsely populated inner shells of those "hollow atoms" survive the approach to the surface [1-5]. As a consequence most of the high initial potential energy of the projectile is available close to the surface plane and is liberated there or in the bulk in close encounters with target atoms and conduction electrons.In all these studies the kinetic energy of the projectiles with respect to the approach to the surface is an important parameter, since this energy is related directly to the interaction times with the surface. Already at an early stage in the study of the scattering of multicharged ions from surfaces it was pointed out that the acceleration of ions due to their image charge interaction sets a lower bound with respect to the interaction energies and interaction times with the surface [6]. Experimental studies on electron emission phenomena at low effective projectile energies (eV domain) have revealed clear indications for such effects [7-9]. Recently we have demonstrated a method to measure directly those image interaction energies [10].Aside from the relevance of image interaction energies for a reliable knowledge of the effective interaction energies/times in scattering experiments, their magnitude also provides important information on the interaction mechanisms. Since the image charge interaction is strongly dependent on the charge state of the particle, image interaction energies reflect the dynamics of neutralization of the multicharged ions in front of the surface plane [6][7][8][9][10][11][12]. Based on the demonstration of the feasibility of our method with ions in low charge states [10], we present here first consequent studies of image charge interaction energies for incident ions ranging from low to relatively high charges.In our experiments we scatter Xe^"*" ions under a grazing angle of incidence ^in^=...

It is shown that the energy loss of multicharged ions in an electron gas has a strong dependence on target electronic screening and the occupation of projectile levels. In our calculations, an enhancement (or decrease) of the energy loss as it depends on the number of vacancies in the inner shells is found for L (or K) shells. Experiments on the charge state dependence of the energy loss of multicharged N ions scattered under grazing incidence condition off an Al (111) surface are explained consistently by our model. [S0031-9007(98)08368-9] PACS numbers: 79.20.Rf, 34.50.Bw, 34.50.DyWhen ions traverse solid matter they lose kinetic energy by electronic excitations and by collisions with lattice atoms ("electronic" and "nuclear" energy loss, respectively). In recent years, the availability of powerful ion sources for highly charged ions resulted in an increasing number of experimental studies on the energy loss of slow multicharged ions ͑y # y 0 Bohr velocity͒ in solid matter. One is faced with the interesting question, how preequilibrium of charge states does affect the energy loss of multicharged projectiles, which is closely related to the charge transfer dynamics.Several experimental studies on the energy loss of multicharged ions in solid targets have been devoted to seek the effect caused by the projectile charge state. Different conclusions have been revealed from experiments via transmission through thin foils [1] or specular reflection from surfaces under grazing incidence [2-4], but no conclusive explanation of the effect in the energy loss due to the presence of inner-shell vacancies has been given. In the case of transmission experiments the preequilibrium length is too short compared to the target thickness [1], while in ion-surface scattering either the authors have concentrated on the final charge state dependence [2] or only two initial charge states have been used [3,4]. Very recently Schenkel et al.[5] reported on energy loss of O q1 , Ar q1 , Kr q1 , Xe q1 , and Au q1 projectiles with velocity y 0.3y 0 transmitted through thin carbon foils, where an enhancement of the energy loss with the initial charge is observed for O q1 , Xe q1 , and Au q1 ions. The authors extract from these data the presence of preequilibrium contributions to the stopping of ions in conducting solids.In a simple intuitive picture, preequilibrium effects have been described by an enhanced effective charge resulting in a higher energy loss as compared to projectiles in low charge states [1,6]. However, screening effects by conduction electrons make the electronic stopping of atomic projectiles a complex problem that cannot be described by making use of a simple picture as an effective charge model [7]. In this respect, information is needed about the effect produced on the electronic stopping by the presence of inner-shell vacancies in the projectile ions [8]. The aim of this paper is to demonstrate that the combined projectile and target electron screening affects the energy loss of multicharged projectiles in a nontrivial m...