D. Schmaus

Surface relaxation and near-surface atomic displacements in the N/Cu(100) self-ordered system

The atomic displacements of Cu atoms induced by nitrogen adsorption on Cu(1 0 0) have been studied by channelling–blocking of swift 4He ions. This study has been performed at two adsorption stages. The first one corresponds to the formation of a dense, two-dimensional, self-ordered array of square-shaped islands covered by nitrogen. The second one corresponds to uniform coverage at saturation. We have determined by nuclear reaction analysis the absolute quantity of nitrogen adsorbed at these two stages. The values obtained, when confronted to previous observations of these stages by low energy electron diffraction and by scanning tunnelling microscopy, demonstrate that nitrogen remains mostly at the sample surface and that the N concentration in bulk Cu could not exceed 1%. However, channelling measurements show that this surface adsorption generates atomic displacements of Cu atoms down to depths of a few ten (1 0 0) interplanar distances. In the mean time, blocking measurements reveal that nitrogen adsorption induces a strong surface expansion: the interplanar distance between the first two (1 0 0) planes increases of about 0.2 A, in contrast with the weak contraction observed on bare Cu(1 0 0) surfaces. This observation supports the hypothesis that, when nitrogen is adsorbed, the surface is submitted to stress variations, from tensile to compressive stress for, respectively, bare and nitrogen-covered surface regions. The surface forces corresponding to such variations have been introduced in molecular dynamics simulations. For coverage leading to self-ordering, these simulations do indeed predict displacements of subsurface Cu atoms. The adjustment of these displacements to those measured by channelling gives the amplitude of the stress variation.

Temperature dependence of the atomic relaxations and vibrations on a stepped surface: a molecular dynamics study of Cu(1,1,19)

We have studied the relaxations and vibrations of atoms near the surface of a (1,1,19) copper crystal. For this purpose, we have performed molecular dynamics simulations using a semi-empirical many-body potential derived from tight binding models. The total displacement field can be described as the sum of a mean surface relaxation and a specific contribution of the steps, which is maximal for step edge atoms (0.08 A) and corner atoms (0.06 A). Whereas step edge atoms relax towards the inner terrace and towards the bulk, corner atoms relax in the opposite direction, leading to vortex-like structures in the displacement field. We demonstrate that, as predicted by continuous elasticity, the displacement field induced by steps is equivalent to the one created by a line of dipoles on a flat surface. In the particular case studied here, the equivalent dipole density is 3.3×10−10 N. The specific relaxations of kink atoms have been calculated. We have also studied the variation of the relaxations as a function of temperature (T). A strong effect is predicted for inner terrace atoms: when increasing T, the contraction of the first interplanar distance, with respect to the bulk value, progressively cancels and turns to an expansion at high T. This is not the case for the specific contraction of step edge atoms that is nearly temperature independent. This latter behaviour is related to very strong longitudinal correlation between vibrations of the step edge atom and of its nearest neighbour inside the terrace. In the same time, whereas the vibrations of inner terrace atoms are found to be isotropic, the ones of step edge atoms are anisotropic, with a larger component along the direction parallel to the terrace plane and perpendicular to the step edge, the other components being the same as for inner terrace atoms.

Influence of intrashell excitation (n=2) on the population of mestastable states of H- and He-like krypton ions in channeling conditions

Abstract We have observed delayed K α photons emitted by 60 A MeV H-like and He-like krypton ions leaving a 37 μm silicon crystal, both for random and 〈110〉 axial alignment of the target. We could extract intrashell (2s→ 2p) excitation probabilities, which are compared with values deduced from PWBA calculations.

Electronic capture and excitation of highly charged channeled ions

Abstract Two aspects of heavy ion channeling are presented. The first aspect is related to the fact that channeled ions interact only with the most loosely bound target electrons. One can take benefit of this feature to study processes such as radiative electron capture (REC) and resonant transfer and excitation (RTE) in a dense quasi-free electron gas. The experimental work, performed at GANIL, devoted to these two processes is described. A possible extension to Nuclear RTE or NEEC (nuclear excitation by electron capture) studies is also described. The second aspect discussed is related to the periodicity of the potential experienced by channeled ions. We show that in a well chosen case this could lead to a significant and detectable coherent excitation of the projectile nucleus.

Lead on Cu(100) and Cu(510): desorption and surface diffusion near the bulk melting temperature of Pb

Abstract The desorption and surface diffusion of Pb, in the submonolayer range, on both (100) and (510) Cu surfaces are studied under ultrahigh vacuum conditions, in a goniometric chamber coupled to a 2.5 MV Van de Graaff accelerator. Absolute Pb surface concentrations are determined by Rutherford backscattering (RBS), using 500 keV 4He beams. The results are presented for two treatment temperatures: 573 and 638 K, i.e. slightly below and above the bulk melting temperature of Pb (600 K). The coefficients for diffusion parallel to the [100] step edges of the (510) surface are, on average, 2.5 times higher than on the (100) surface, indicating preferential occupation of step sites by Pb atoms. In the whole range studied (between 0.1 and 1 Pb monolayer), the desorption rate and the diffusion coefficients increase markedly with the concentration. Also, sharp variations of the diffusion coefficient on Cu(510) are observed for Pb coverages around 0.3 monolayer and may be related to faceting, which is known to take place at this Pb concentration.

Recent questions about the excitation mechanism at intermediate velocity

Abstract For both fundamental reasons and potential applications, the process of K-excitation by heavy ions at intermediate velocity has been re-investigated experimentally and theoretically and the main results are described here. The nature and the origin of the so-called “saturation effect” at high exciting charge is not yet elucidated. The importance of K-excitation is outlined in connection with problems of radiation damage.

Channelling experiment with a cooled and decelerated H-like gold ion beam extracted from a storage ring

We have performed a channelling experiment in which a ion beam, extracted by radiative recombination from the storage ring ESR at GSI after deceleration and cooling, was directed onto a thin silicon crystal. The directional properties of channelling have been used to test the quality of the extracted beam. An upper limit of the beam emittance has been measured to be via x-ray measurements. Coincidence measurements between photons and ions transmitted after capture of one electron in the crystal have allowed us to study L-REC and the associated emission.

X-rays and inner-shell processes with heavy ions channeled in thin crystals

We present some of the recent developments on heavy-ion channeling experiments, in the framework of X-ray emission and inner-shell processes. We discuss the possibility to characterize the very low electron densities sampled by hyperchanneled ions, and we report on an attempt to observe exotic trielectronic recombination.