|Aufstellung der Publikationen der Arbeitsgruppe 1995|
Aufgeführt sind Arbeiten, die im Jahr 1995 publiziert und zur Veröffentlichung eingereicht oder angenommen worden sind. Sonderdrucke oder Preprints können von den Autoren oder im Sekretariat des Lehrstuhls angefordert werden (Postanschrift: Humboldt-Universität zu Berlin, Institut für Physik, Physik der Grenzflächen und dünnen Schichten, Newtonstraße 15, 12489 Berlin, Tel.: (+49 30) 20 93 - 78 91, Fax: - 78 99).
Multicharged xenon ions are scattered with keV energies from a LiF(100) surface under a glancing incidence. From the angular distributions of scattered projectiles, we deduce the existence of an attractive force acting on the projectiles on the incident path. We interpret this as the dielectric response of the insulator due to the presence of an ion. The interaction energies gained on the incident trajectory are reproduced by an "overbarrier" model of stepwise capture of electrons from the fluorine 2p band of LiF into Rydberg levels of the projectile.
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 O- 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.
3. T. Hecht, C. Auth, A. G. Borisov, and H. Winter
We have scattered noble gas atoms and ions from a clean and flat LiF(100)-surface under a grazing angle of incidence and observed for He+- and Ne+-projectiles a substantial survival in the collision. We interpret our findings in terms of a suppression of the Auger-neutralization for these ions in the interaction with the surface of the insulator. Due to the high binding energies of valence band electrons and the large energy gap for LiF Auger transition rates seem to be clearly reduced in comparison to those rates for metal targets.
4. H. Winter, C. Auth, and A. G. Borisov
We have studied charge exchange between fast atoms/ions and the surface of an insulator. Based on investigations on the formation of negative hydrogen, oxygen, and fluorine ions in the grazing scattering from a clean and flat LiF(100)-surface we discuss specific features of the interaction of fast atomic projectiles with the surface of an insulator. Under specific kinematic conditions large fractions of negative ions are observed.
5. A. G. Borisov and H. Winter
A theoretical study on the formation of the Na I 3s-term in grazing scattering of Na+-ions from the (111)-surface of an aluminum crystal and a thin film is performed. For the crystal-surface we find quantitative agreement between experimental and theoretical results. For a thin film the quantization of the electronic states in the direction normal to the surface strongly influences the charge-exchange process. The dependence of the neutral fractions in the scattered beam on the projectile velocity is predicted to be clearly different for electron capture from the film and the semi-infinite crystal.
6. A. G. Borisov, H. Winter. G. Dierkes, and R. Zimny
We performed detailed theoretical and experimental studies on the neutralization of Na+-ions with keV energies in grazing scattering from a clean and flat Al(111)surface. Our data are described well by model calculations based on the non-perturbative CAM method. From the analysis of our experiments we deduce the effective binding energy of the Nal 3s-level in front of the metal surface.
Protons with energies ranging from about 6 to 30 keV are scattered under grazing angles of incidence from a flat and clean Al(111)-surface. We observe angular and energy distributions of the scattered projectiles with high resolution and find for the mean energy loss a pronounced increase with projectile energy and a slight decrease for an increasing angle of incidence. We present model calculations based on the assumption that the energy loss is predominantly due to binary collisions with conduction electrons. These calculations reproduce main features of our data fairly well.
8. H. Winter
Collisions of slow multicharged and highly charged ions, i.e. ions with kinetic energies up to some 10 keV, with atoms in the gas-phase and with solids have been studied in detail over the last years. A brief review is presented on some typical experiments and their results which make use of the specific features of highly charged ions in the fields of atomic spectroscopy, ion-atom-collisions, and ion-solid interactions. In particular recent progress in the understanding of the interaction mechanisms of highly charged ions with solid surfaces will be discussed.
9. C. Auth and H. Winter
We have measured energy loss spectra of 100 keV protons interacting with noble gas atoms in small angle scattering under single collision conditions. We obtain from our experiments with high angular and energy resolution the mean energy loss and the energy straggling as function of the angle of scattering ranging from 0.3 to 6 mrad.
10. A. G. Borisov and H. Winter
We report on a theoretical study on the formation of negative hydrogen ions in the scattering from a thin aluminum film. In contrast to a semi-infinite metal electronic states of the film show pronounced effects of quantization along the direction of the surface normal. We predict that this quantization has specific effects on the formation of H--ions in the scattering from thin films.
11. A. G. Borisov, R. Zimny, D. Teillet-Billy, and J.
The nonperturbative Coupled Angular Mode (CAM) method is applied to the calculation of the widths and the energies of the multy charged ion states in front of the Al surface. Within n manifolds we observe formation of the hybrides with very different widths. The hybridisation of the states is analysed. Importance of the linear Stark effect is demonstrated. Scaling of the widths of the states is observed.
12. A. G. Borisov and U. Wille
The reliability of a first-order model that is in use for calculating the widths of resonantly broadened atomic levels near metal surfaces is examined, in particular for large atomic quantum numbers and core charges. The model employs a simplified electronic potential in which image-charge effects are disregarded. Orbital hybridization is simulated by describing the atomic states in terms of hydrogenic wave functions in the parabolic (Stark) representation. For typical cases, first-order resonance widths are compared to widths calculated from the nonperturbative coupled-angular-mode method which uses a fairly realistic electronic potential including the classical image potentials. Good overall agreement is achieved between first-order and nonperturbative results. A plausible explanation for this agreement is given.
13. A. G. Borisov and R. Zimny
A study of the angular momentum orientation of excited Na*(3p)-atoms by grazing scattering of Na+-ions from an Al surface is presented. The CAM method, previously used for the nonperturbative treatment of dynamical resonant electron transfer in grazing-atom surface collisions [1, 2], is used for the study of this polarization problem. Following the dynamical approach of Brako [3, 4] density matrix elements are calculated within the 3p-subspace of Na. A pronounced orientation of the orbital angular momentum perpendicular to the scattering plane is obtained. And for the first time a theoretical calculations has yielded a decrease of this atomic orientation with decreasing ion-beam velocities below 0.4 Bohr velocity. Overall agreement of the theoretical results with experimental observations is found as well.
14. W. Frentrup, A. Mertens
Buried layers of fluorine were prepared by ion implantation technique. Degree of amorphization and atomic redistribution after thermal treatment were measured by RBS and SIMS, respectively. Fluorescence NEXAFS with synchrotron radiation was used to study electronical structural changes at the F 1s edge. Both, increase of implantation dose and annealing, cause a shift of the F 1s absorption edge to higher energies and a formation of additional peaks at 690, 695, and 716 eV. This behaviour is referred not only to the increase of crystal defects, but also to the interaction between neighboured fluorine and silicon atoms.
15. E. C. Paloura, A. Mertens, K. Holldack
The effect of ion implantation in the microstructure of thin Si3N4 films is studied with Extended X-ray Absorption Fine Structure (EXAFS) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) measurements at the N-K-edge. The stoichiometric Si3N4 films were subjected to implantation with Ar+ (70 keV, 5x1016 cm-2) or N2+(60 keV, 1x1017 cm-2)ions, at 77K and 300K. Analysis of the NEXAFS spectra indicates that ion implantation creates N dangling bonds which introduce a characteristic resonance line (RL) localized at 0.67 eV above the conduction band edge, and a red shift of the absorption edge (D E ). The intensity of the RL as well as the shift D E , which takes values in the range 0.2 to 0.4 eV, depend on the projectile and the substrate temperature during implantation. Finally, the EXAFS data demonstrate that implantation modifies the microstructure down to the nearest neighbour shell, as that is indicated by the reduction of the coordination number from 3 to 2. The presented experimental evidence identifies the origin of the RL with the N dangling bond defect localized on an under-coordinated N atom.
16. M. Rösler
In the particle-induced kinetic electron emission from solids the role of different excitation and scattering processes is of special interest from both the theoretical and the experimental point of view. Starting from a microscopic description of the ion-induced kinetic electron emission based on the transport equation formalism, calculations of the emission characteristics were performed for different nearly-free electron metals. Depending on the impact energy the relative importance of different excitation mechanisms will be discussed for Mg and Be in comparison with the results for the standard material Al. Some statements about the importance of the excitation processes related to conduction and core electrons obtained for Al can be generalized to other simple metals. However, there are characteristic features related to the different strength of plasmon damping in these materials. The role of different scattering processes (elastic, inelastic) which determine the transport of excited electrons towards the surface will be discussed in relation to the special features shown by the various excitation processes.
17. S. Parhofer, R. Pfandzelter, and M. Potthoff
CuL3M4,5M4,5 Auger spectra excited by impact of 1.0 MeV protons on Cu(110) were measured. In the case of grazing incidence angles, a shift of the Auger line to higher kinetic energies was observed. This is attributed to a surface effect. The observed shift is interpreted within the Cini-Sawatzky theory. Taking into account the surface core-level shift and the surface shift of the center of gravity of the valence band only, is not sufficient to explain the observed shift of the Auger line. We find evidence that the shift is partly due to a change of the on-site Coulomb interaction U at the very surface. A rough estimation shows U to be increased by less than 0.6 eV at the surface when compared with the bulk.
18. R. Pfandzelter and S. Parhofer
We measured the fine structure of the Cu L3M4,5M4,5 Auger line excited by impact of MeV protons upon a Cu(110) surface. In case of a grazing incidence, a shift of the Auger line by 0.4 eV to higher kinetic energies is observed. This is attributed to a surface effect: The grazingly incident protons undergo semi-planar channeling at the surface and mainly excite Auger electrons at topmost surface layer atoms. Computer simulations which take into account surface steps corroborate this interpretation.
19. R. Pfandzelter and J. Landskron
We studied the growth of Ag on Cu(111) by electron- and proton-induced Auger electron spectroscopy. The Auger signal versus coverage plots indicate a growth in a monolayer-by-monolayer fashion. In case of grazingly incident protons, additional information on the real mesoscopic structure in the submonolayer coverage range could be obtained by comparison of the experimental results with computer simulations. We found good agreement for Ag islands growing from Cu step edges.
20. S. Gelfort, H. Kerkow, R. Stolle, V. P. Petukhov,
E. A. Romanowski
The electronic stopping of heavy ions with 2 £ Z1 £ 36 in <110>-direction of a 350 nm thick silicon single crystal was measured for velocities below Bohr velocity. Large oscillations in the electronic stopping power are observed for best-channeled ions of the same velocity, whereas for ions emerging from the foil some degrees to the channel axis the oscillations are strongly reduced.
21. A. G. Borisov, U. Wille
Transition rates for resonant electron transfer between highly charged ions and metal surfaces are calculated within a simple first-order model and within the nonperturbative coupled-angular mode (CAM) method. In the first-order model, image potentials are disregarded and hybridization effects are simulated by describing the ionic orbitals in terms of hydrogenic wave functions in the parabolic (Stark) representation. The CAM method employs a fairly realistic electronic potential that includes the classical image potentials. For typical cases, the deviations between first-order and CAM transition rates are such that the associated transition distances agree, on average, within one atomic unit. The reason for this agreement is indicated.
22. M. Maazouz, R. Baragiola, A. Borisov, D. Teillet-Billy,
V. A. Esaulov, S. Lacombe, J. P. Gauyacq, and L. Guillemot
We present the results of an experimental and theoretical investigation of H- formation in collisions of 1 to 4 keV positive and negative hydrogen ions with clean Al surfaces. The scattered ion fractions were investigated in a large angular range extending from 2 deg to 40 deg with respect to the surface plane. This allowed us to investigate the characteristics of the resonant charge transfer process for conditions of fairly high velocities normal to the surface, when small atom surface distances play an important role. Results of theoretical calculations could thus be tested in a more stringent manner than in previous highly grazing incidence experiments of Wyputta et al.
23. E. C. Paloura, A. Mertens, W. Frentrup, U. Döbler,
A. Knop, W. Braun
Buried SiNx films. formed by ion implatation of 35 keV N+ ions in Si, in the dose range 2 x 1017 - 2 x 1018 ions/cm², are studied with EXAFS and NEXAFS. The N-rich nitrides are characterized by N dangling bonds which introduce a characteristic resonance line in the NEXAFS spectra and cause a nominal reduction in the coordination number. All the buried nitrides are characterized by bond lengths which are comparable to those in the reference nitride.
24. T. Hecht, C. Auth, and H. Winter
We have scattered fast He-atoms with keV-energies under a grazing angle of incidence from a clean and flat Al(111)-surface. From the analysis of the angular distributions for scattered projectiles and for scattered light of a He-Ne-laser we find evidence for a reflection of the fast atoms from single terraces of the crystal surface. From our experiments we deduce in a simple manner an averaged width for terraces forming a stepped surface,
25. H. Winter, A. Mertens, and C. Auth
Fast positive ions and atoms with energies ranging from some 100 eV to 100 keV are scattered under a grazing angle of incidence from a clean and flat (100)-surface of a monocrystalline KI-sample. We observe for halogen projectiles an almost complete negative ion conversion, i.e. negative ion fractions up to 98.5 %. Our data show a characteristic dependence on the projectile velocity which provides important additional information on the mechanisms of formation of negative ions in the scattering from the surface of an insulator.
26. T. Igel, R. Pfandzelter, and H. Winter
We report on grazing scattering of 25 keV He+-ions during epitaxial growth of Cr on Fe(100). The angular distributions and intensity of the scattered particles depend on the coverage in an oscillatory way, which is due to the periodical change of the surface morphology in two-dimensional growth. At high growth temperatures (600 K), pronounced oscillations which persist with little decay in the amplitude indicate (almost) ideal layer-by-layer growth; at low temperatures (350 K), strongly damped oscillations and low intensities of scattered projectiles are due to transient layer-by-layer growth. The angular distributions are well described by Monte-Carlo simulations using a classical description for the trajectories. This enables one to derive detailed information on the morphology of the growth front, like, e.g., the mean island distance.
27. M. Rösler
In the secondary electron emission (SEE) from solids the role of different excitation processes is now as ever of special interest from both the theoretical and the experimental points of view. Depending on the primary energy the relative importance of different excitation mechanisms related to conduction as well as core electrons will be discusses for different simple metals. So far first principles results are available only for Al for primary energies up to 10 keV. Starting from a microscopic description of the SEE based on the transport equation formalism, calculations were performed for other nearly-free-electron metals (Mg, Be) up to primary energies used in scanning electron microscopy. In this way it is possible to obtain more general statements about the role of different excitation processes responsible for SEE. Special attention is devoted to the contribution of emitted electrons stemming from the excitation of conduction electrons by decay of bulk plasmons generated by the primary electron. The different strength of this excitation mechanism in Al, Mg, and Be is related to the different magnitude of the plasmon damping in these metals.
28. M. Rösler
The characteristics of electron emission induced by charged particle beams (electrons, ions) can be calculated from first principles only for simple metals. The microscopic description of the particle-induced kinetic electron emission based on the transport equation formalism allows the investigation of different excitation and scattering mechanisms which govern the electron emission of these metals. Besides the excitation of single conduction and inner shell electrons, the indirect excitation of conduction electrons by plasmon decay and Auger processes is taken into account. Results of detailed calculations for Mg and Be will be presented using the same basic models as in the case of Al. An extended range of impact energies for electron and proton bombardment is considered. Depending on the impact energy, some statements obtained for Al concerning the relative importance of different excitation mechanisms have to be modified for Mg and Be. Special attention is devoted to the electron emission related to the excitation of conduction electrons via interband transitions by decay of bulk plasmons generated by the impinging particle. It is shown that for both emission phenomena this excitation mechanism is of importance in the whole range of impact energies. In the case of proton-induced kinetic electron emission it can be shown that at proton impact energies slightly above the threshold for plasmon generation there is a direct relation between plasmon dispersion and the energetic position of the so-called plasmon shoulder in the energy distribution of emerging electrons.
29. A. Dubus and M. Rösler
It has been evidenced experimentally that, for proton impact on a thick solid target, the ratio L of the backward electron yield g B and the electronic stopping power (dE/dx)e is approximately constant over a large energy range (a few keV to several MeV). The macroscopic theoretical models (Sternglass, 1957; Schou, 1980; ...) assume the constancy of this ratio. However, from a microscopic point of view, there is no obvious reason for that. We present in this paper a microscopic calculation (microscopic cross sections and Monte Carlo simulation method for the transport) for proton induced electron emission from aluminium and amorphous carbon. The electron yields and the factors L from thick and thin (forward and backward emissions) targets are calculated and compared to experimental results.
30. C. Auth and H. Winter
Multicharged Xeq+-ions (q < 20) are scattered with keV-energies under a grazing angle of incidence from the surface of two different insulators: LiF(100) and KI(100). From the analysis of angular distributions for scattered projectiles we deduce energy gains caused by the dielectric response of the target material due to the presence of the external charge ("image charge interaction"). A direct comparison of the interaction energies gained by the projectiles on the incident path with the two different targets is consistent with the assumption of a dominant population of Rydberg-levels ("hollow atom") in front of insulators for grazing ion-surface scattering.
31. A. G. Borisov, V. Sidis, and H. Winter
The mechanism mediating electron transfer from an alkali halide surface to an atom by bringing diabatically the relevant atomic and surface energy levels into near resonance is elucidated. The mechanism is supported by ab initio calculations on a model F/LiF(100) system where all sites of the crystal lattice but one, the active site, are represented by (polarizable) point charges. The electron transfer interaction between the atom and the active site of the surface is computed and used in dynamics calculations of negative ion formation in a sequence of binary collisions.
32. K. Holldack, M. Grunze, M. Kinzler, H. Kerkow, and
C. R. Brundle
The radiation induced changes caused by both low energy Ar+ ion bombardment and high energy H+ ion bombardment on Langmuir-Blodgett (LB) films of cadmium arachidate were investigated using a commercial photoelectron microscope installed at the German synchrotron facility BESSY. For the low energy situation, XPS imaging was performed using damage induced differential charging as the contrast mechanism. The high energy H+ bombardment produced less damage and no resolvable charging shift, so this contrast mechanism cannot be used for imaging. However, chemical shifts in the C NEXAFS (near edge X-ray absorption fine structure), caused by the H+ bombardment, are sufficiently large to use as a contrast mechanism. NEXAFS, of course, requires tunable radiation and hence spectromicroscopy in conjunction with synchrotron radiation offers the possibility for imaging chemical changes in organic films which are not observable using fixed energy laboratory sources. In principle, molecular orientation changes are also imageable, as the relative intensities of different features in a NEXAFS spectrum often vary with orientation.
33. R. Pfandzelter, T. Igel, and H. Winter
1 and 2 ML Cr films have been grown on Fe(100) under layer-by-layer conditions and studied using Auger electron spectroscopy induced by electrons and grazingly incident protons. Exploiting the different sampling depths for proton- and electron-excitation, we could quantitatively evaluate the layer-dependent concentration profiles of Cr and Fe near the film surface and their evolution in time. For a coverage of 1 ML, more than half the deposited Cr atoms have undergone place exchange with Fe substrate atoms, which results in a mixed first and second layer. For 2 ML, the first and second layer consist predominantly of Cr. Long-time measurements show that as grown 1 ML films are not stable.