P. Roncin

Grazing incidence fast atom diffraction: An innovative approach to surface structure analysis

An alternative diffraction technique, based on grazing incidence scattering of high energy atoms, is applied to surface structure determination of crystalline surfaces. This technique, named GIFAD for grazing incidence fast atom diffraction, uses the same geometry as reflection high energy electron diffraction but is less invasive, more surface sensitive, and readily interpretable quantitatively. We present here a demonstration of this approach on a prototypical II–VI compound, ZnSe(001). Besides providing lattice parameter with high accuracy, we show that GIFAD gives straightforward access to the surface valence electron density profile, allowing clear identification of an electron transfer from Zn to Se.

Charge exchange of low energy ions in thin carbon foils. II. Results for ions of B, C, F, Ne, Na, Si, S, Cl, Ar, K, and Fe

In order to calibrate a novel type of time‐of‐flight mass spectrometer to be flown in the solar wind, we have continued our investigation of the charge exchange of low energy ions passing thin carbon foils. We analyzed elements with widely different chemical properties: Ions of B, C, F, Ne, Na, Si, S, Cl, Ar, K, and Fe in the energy range 0.5–3 keV/u were passed through carbon foils with thicknesses between 1.1 and 10 μg/cm2, and their charge state distributions and residual energies were determined. It was found that (1) the charge state distribution behind the foil is independent of the charge of the incident projectile, (2) isotopes show the same charge exchange properties at equal velocities as we have found previously, (3) at the lowest energies the charge state distribution is no longer a function of the residual energy alone but depends on both residual energy and foil thickness, (4) probable differences in chemical properties between the front and back surfaces of the foil have no detectable influ...

2π spectrometer: A new apparatus for the investigation of ion surface interaction

A new spectrometer for studying ion surface interaction is described. This apparatus is built around a secondary electron and ion detector with a very large acceptance angle and made of 16 individual microchannel plate detectors located on a half sphere. A simultaneous detection of the scattered projectiles with an additional position sensitive detector allows measurements of the correlation between all these particles using a multicoincidence technique. With this spectrometer, a large variety of measurements are possible such as the energy spectra of the secondary electrons as well as the statistics of the number of ejected electrons, the scattering pattern of the reflected projectiles and their charge‐state distribution, the analysis of the sputtered ions. Some examples are given concerning the impact of multiply charged ions on a LiF single crystal. The dependence of the secondary electron multiplicity as a function of the charge state, of the surface channeling condition, and of the scattering angle o...

Determination of the geometric corrugation of graphene on SiC(0001) by grazing incidence fast atom diffraction

We present a grazing incidence fast atom diffraction (GIFAD) study of monolayer graphene on 6H-SiC(0001). This system shows a Moire-like 13 × 13 superlattice above the reconstructed carbon buffer layer. The averaging property of GIFAD results in electronic and geometric corrugations that are well decoupled; the graphene honeycomb corrugation is only observed with the incident beam parallel to the zigzag direction while the geometric corrugation arising from the superlattice is revealed along the armchair direction. Full-quantum calculations of the diffraction patterns show the very high GIFAD sensitivity to the amplitude of the surface corrugation. The best agreement between the calculated and measured diffraction intensities yields a corrugation height of 0.27 ± 0.03 A.

Surface analysis with grazing incidence fast atom diffraction (GIFAD)

Grazing collisions at surfaces offer rather contrasted conditions. For well ordered flat surfaces, the scattering is spread among several lattice sites, each of which produces only a tiny elementary deflection. If, in addition, the atomic projectile is aligned along a crystallographic direction, the surface appears as made of parallel furrows or as a washboard which act as a diffraction grating for the atomic wave. We will show that the analysis of characteristic diffraction pattern recorded on a position sensitive detector located downstream allows a sensitive measure of the shape of the surface electronic density. A modified Debye Waller factor is proposed to explain the observed diffraction signal.

Differential Cross-Sections for One- and Two-Electron Capture by Highly Charged Ions (N7+, O7+, O8+, Ne7+, Ne8+) at Low keV Energies

This letter reports the first measurements of differential cross-sections (DCS) for one- and two-electron capture by highly charged ions at low keV energies. These measurements have been achieved using a new type of spectrometer with a design based on a two-dimensional position-sensitive detector. A comparison of the shape of DCS for, respectively, one- and two-electron capture processes provides information about mechanisms for two-electron capture. Our results concerning capture by highly charged ions (q = 7,8) clearly indicate that the two electrons are captured successively during the collision.

Dynamic grazing incidence fast atom diffraction during molecular beam epitaxial growth of GaAs

A Grazing Incidence Fast Atom Diffraction (GIFAD) system has been mounted on a commercial molecular beam epitaxy chamber and used to monitor GaAs growth in real-time. In contrast to the conventionally used Reflection High Energy Electron Diffraction, all the GIFAD diffraction orders oscillate in phase, with the change in intensity related to diffuse scattering at step edges. We show that the scattered intensity integrated over the Laue circle is a robust method to monitor the periodic change in surface roughness during layer-by-layer growth, with oscillation phase and amplitude independent of incidence angle and crystal orientation. When there is a change in surface reconstruction at the start of growth, GIFAD intensity oscillations show that there is a corresponding delay in the onset of layer-by-layer growth. In addition, changes in the relative intensity of different diffraction orders have been observed during growth showing that GIFAD has the potential to provide insight into the preferential adatom attachment sites on the surface reconstruction during growth.

Electron emission in collisions of slow, highly charged ions with gases and solid surfaces

Abstract We measured electron spectra produced in O 6+ + He collisions and by N 6+ , O 7+ , Ne 9+ , and Ar q + ( q = 9, 13, 14 and 16) incident on solid surfaces of Cu and C under 10 ° with energies from 10 q to 20 q keV. Auger electrons were detected at various emission angles including 0° and 180° with respect to the incident beam. For the He gas target the angular distribution of oxygen L-Auger electrons is found to be isotropic between 10 ° and 140 ° but enhanced at 0 °. In ion-surface collisions, multiple electron capture is studied. By analyzing the Doppler shift of the projectile Auger electrons, the L-Auger electrons of argon were found to be emitted from ions incident on the surface and the K-Auger electrons of neon were observed to be emitted from reflected ions.

Elastic and inelastic diffraction of fast atoms, Debye-Waller factor, and Mössbauer-Lamb-Dicke regime

The diffraction of fast atoms at crystal surfaces is ideal for a detailed investigation of the surface electronic density. However, instead of sharp diffraction spots, most experiments show elongated streaks characteristic of inelastic diffraction. This paper describes these inelastic profiles in terms of individual inelastic collisions with surface atoms taking place along the projectile trajectory and leading to vibrational excitation of the local Debye oscillator. A quasi-elastic regime where only one inelastic event contributes is identified as well as a mixed quantum-classical regime were several inelastic collision are involved. These regimes describe a smooth evolution of the scattering profiles from sharp spots to elongated streaks merging progressively into the classical diffusion regime.

A 2 π steradian low energy electron spectrometer for probing multiply charged ion surface interaction

We describe here a new type of multi-detector designed to study the interaction of highly charged ions (HCI) with surfaces. This interaction gives rise to the emission of a large number of slow electrons. For example, the impact of a single Th71+ ion on a gold target produces a mean number of 250 “slow” electrons (E < 60 eV) H. Kurz et al., Phys. Rev. A 49 (1994) 4693 [1]. On the other hand, most of the works have focused on the analysis of the high energy electrons originating from inner shell transitions in the partially neutralised projectile. In between pure statistics or single spectroscopy, the goal of our experiment is to focus on the analysis of the low energy electrons which are by far the more numerous and to measure their energy and angular distribution as well as the correlation between these quantities. For this purpose, we have designed and built a 2 π detector made of 16 independent detection units, made of two multichannel-plates (MCP), located on a half sphere of 60 mm radius surrounding the solid target. If the impact time of the projectile on the target is known, then measurement of the time of flight (TOF) corresponding to a hit on a particular detector gives information on the nature, the energy and on the emission angle of the detected particle. For example: • - Photons are expected during the first nanosecond. • - Electrons TOF should range between a few ns to a few 100 ns depending on their energy. • - Ions TOF are in the μs scale and depend both on their energy and mass.