W. Hofer

Auger neutralization of multiply charged noble gas ions at a tungsten surface

Abstract Neutralization processes for slow doubly and triply charged noble gas ions impinging on clean polycrystalline tungsten are studied by observation of the resulting ejected electron energy distributions. A sequence of Auger de-excitation and Auger neutralization processes is demonstrated to occur and therefore strong evidence for participation of one-electron like transitions is presented. In the special case of Ne 2+ impact this may yield straightforward information on the density of states of the uppermost surface layer.

Preferential sputtering and segregation reversal: (100) and (110) surfaces of Pt25Ni75 single crystal alloys

Abstract Low energy ion scattering (LEIS) and Auger electron spectroscopy (AES) are applied to determine the surface composition of Pt25Ni75(100) and Pt25Ni75(110). After annealing at 970 K the topmost atomic layer of Pt25Ni75(100) is observed to be enriched in Pt to a value of 39 at%, whereas the first monolayer of Pt25Ni75(110) is found to consist nearly exclusively of Ni atoms. The second layer of the (110) surface shows strong Pt enrichment. The bombardment with low energy ions at room temperature induces the preferential sputtering of Ni atoms. The LEIS results indicate the superposition of the effects of preferential sputtering and segregation. The dependence of the surface composition on the annealing temperature is rather similar for both Pt25Ni75 crystals under investigation though the segregating component differs. Segregation is found to occur at rather low annealing temperatures, whereas high annealing temperatures are necessary to allow thermodynamic equilibration by bulk diffusion.

A new approach to interface segregation: Surface dangling and interatomic bond effects in binary alloys

First a new theory and method of calculation, describing also surface interatomic bonds with environmental atoms is outlined. This theory is termed the MTCIP-1A (modern thermodynamic calculation of interface properties-first approximation). The method permits the calculation of the X1 thermodynamic equilibrium composition (given by the mole fraction of a selected component, e.g. the solute component) of the topmost atomic sublayer of a binary solid. The value of X1 is shown to be determined by the following factors: the “mixing”, the “non-bonding”, the “bonding” and the “real mixture” factors. The first three are described here more deeply. The numerical calculations are shown in detail for an AuPd alloy with a bulk Au content of 25 at%, with (100) surface orientation, with a temperature T = 773.2 K and under an environmental oxygen partial pressure PO2 = 5 × 10−10Pa. Further results are outlined (in appendix 6) for T = 293.2 K and PO2 = 5 × 10−5Pa (much stronger oxidizing conditions). The X1Au value at% in the first case (Au segregation) while it is practically 0 at% in the second case (Pd segregation). Results on this alloy, obtained by ISS (ion scattering spectroscopy) are used for comparison. The initial Au content of the surface (after sputtering) was 29 at%. After 15 min. of equilibration in the first case 75 at% was measured (Au segregation). In the second case just 26 at% was obtained (Pd segregation compared with the initial composition). The calculated X1Au value is close to the experimental one at 773.2 K. At T = 293.2 K the agreement seems to be only qualitative. This is due to the slow approach to equilibrium (from 29 to about 0 at% of Au) by diffusion at such a low temperature and to the relatively small time of equilibration. Several other cases giving good agreement between theory and experimental results are found in other works mentioned here.

Surface segregation of PtNi alloys : comparing theoretical and experimental results

SummaryThe PtNi system is an interesting example of orientation-dependent segregation behaviour. Experimental and theoretical results of different authors on surface segregation of PtNi alloys are compared. The quite different results from the various theoretical predictions show that only very few methods (e.g. the tight-binding Ising model with an area-preserving map technique, the embedded atom method, or a thorough thermodynamic description) can be used successfully for describing the PtNi system. A multilayer thermodynamic description is used to study the influence of the different effects (negligible surface free energy difference, and the competition between an ordering and a size effect) on the composition profile. For comparing experimental results, the influence of preferential sputtering and annealing at different temperatures as well as environmental influence (e.g. due to carbon impurities or oxygen exposure), which might considerably change the measured values, are discussed.

On the surface composition of Pt10Ni90(110): a low energy ion scattering study

Abstract Low energy ion scattering spectroscopy is applied to determine the surface composition of Pt10Ni90(110). Measurements performed at different scattering geometries allow us to distinguish between scattering from first and second monolayer atoms. Trajectory simulations including neutralization allow the determination of first and second monolayer composition. After annealing at 970 K the topmost atomic layer is found to consist nearly exclusively of Ni atoms, whereas the second monolayer shows a strong Pt enrichment. This oscillating behaviour of the composition depth profile is observed after bombarding the surface with ions as well. After thermal treatment distinct differences in the temperature dependence of the composition between the first and second monolayer are found.

Investigation of Pt25Ni75(111): preferential sputtering and surface segregation

The surface composition of Pt25Ni75(111) has been determined by ion scattering spectroscopy and Auger electron spectroscopy. The surface layers are found to be enriched in Pt due to preferential sputtering. Thermal treatment induces further segregation of Pt at the topmost atomic layer. From the different sampling depths of ISS and AES composition profiles are deduced, which show a strong dependence on the temperature during thermal treatment. At an annealing temperature of 770 K the thermally induced Pt segregation is determined by the composition of the sputter induced altered layer being enriched in Pt. However, at an annealing temperature of 970 K the sputter induced altered layer has disappeared and the Pt segregation at the topmost atomic layer is determined by the bulk composition.

The accuracy of quantitative LEED in determining chemical composition profiles of substitutionally disordered alloys : a case study

We explore the accuracy of chemical composition profiles of substitutionally disordered alloys determined experimentally by LEED (low-energy electron diffraction) I(E) analysis. We analyse experimental I(E) spectra of pure Rh(111) for its known chemical composition by comparing them to calculations assuming a substitutionally disordered PtxRh1−x alloy surface. The layer concentrations known to be 100% Rh are reproduced with a maximum error of 8% when the Pendry R-factor (RP) is employed. This error is considerably smaller than estimated by error bars derived from the variance of RP. We argue that the same accuracy can be expected for compositional depth profiles to be determined for alloys exhibiting weak chemical order and negligible lattice distortions such as PtxRh1−x.

Embedded-atom method calculations applied to surface segregation of PtNi single crystals

Abstract The embedded-atom method, a model for the calculation of various crystal and alloy properties, has also been applied to the study of surface segregation phenomena. We employ this formalism for calculating surface segregation on PtxNi1−x single-crystal low-index faces which are known to show an orientation-dependent segregation behaviour. Pt enrichment in the topmost layer and alternating segregation profiles for the (100) and (111) surfaces are found to be in accordance with experimental data. The results for the (110) surface, showing a different behaviour, are discussed. In addition, surface relaxation is calculated and found to correspond with experimental results.

Pt25Ni75(100) and (110) single crystals: preferential sputtering and segregation reversal

Abstract After annealing at 970 K low energy ion scattering (LEIS) reveals the topmost atomic layer of Pt 25 Ni 75 (100) to be enriched in Pt to a value of 39 at%, whereas for Pt 25 Ni 75 (110) the first monolayer is found to consist nearly exclusively of Ni atoms and the second layer shows strong Pt enrichment. LEIS measurements performed after ion bombardment at room temperature indicate the superposition of the effects of preferential sputtering of Ni atoms and segregation. Segregation is found to start at rather low annealing temperatures, whereas high annealing temperatures are necessary to allow thermodynamic equilibration by bulk diffusion. Most of the theoretical models fail to reproduce the orientation dependent change in the segregating component of the PtNi system. One of the few successful approaches is a thorough thermodynamic description which is used here to calculate the composition profile and to discuss the segregation behaviour. Good agreement between measurement and calculation is found.

Apparent cross sections for metastable ion production by electron impact

Fractions of long-lived excited species in noble-gas ion beams which were produced by various methods have been determined by virtue of slow ion impact induced electron emission from clean polycrystalline tungsten. Investigations with ion beams produced by single electron impact on atoms permitted the determination of cross sections for the production of singly charged metastable ions of the noble gases Ar, Kr and Xe. The cross sections obtained increase steeply from threshold up to their maximum values of about 10-17 cm2 within 1.5 time threshold energy. Processes responsible for production of these metastable ions are discussed and the method applied is compared with other techniques for metastable ion detection.