Anomalous KCl(001) Surface Corrugation from Fast He Diffraction at Very Grazing Incidence.

We present theoretical and experimental evidence of an anomalous surface corrugation behavior in He-KCl(001) for incidence along ⟨110⟩. When the He normal energy decreases below 100 meV, i.e., He-surface distances Z>2 Å, the corrugation unexpectedly increases up to an impressive ≳85%. This is not due to van der Waals interactions but to the combination of soft potential effects and the evolution of He-cation and He-anion interactions with Z. This feature, not previously analyzed on alkali-halide surfaces, may favor the alignment properties of weakly interacting overlayers.

Transient quantum trapping of fast atoms at surfaces.

We report on the experimental observation and theoretical study of the bound state resonances in fast atom diffraction at surfaces. In our studies, the 4He atom beam has been scattered from a high-quality LiF(001) surface at very small grazing incidence angles. In this regime, the reciprocal lattice vector exchange with the surface allows transient trapping of the 0.3-0.5 keV projectiles into the quasistationary states bound by the attractive atom-surface potential well which is only 10 meV deep. Analysis of the linewidths of the calculated and measured resonances reveals that prior to their release, the trapped projectiles preserve their coherence over travel distances along the surface as large as 0.2 µm, while being in average only at some angstroms in front of the last atomic plane.

High resolution imaging of superficial mosaicity in single crystals using grazing incidence fast atom diffraction.

A new table top technique is used to simultaneously analyze the local morphology of crystalline surfaces as well as the misalignment of large scale domains at the topmost surface layer. The approach is based on fast atom diffraction at grazing incidence (GIFAD); the diffraction pattern yields the structural characteristics and the topology of the surface electronic density with atomic resolution. If superficial mosaicity is present, diffraction patterns arising from each mosaic domain can be distinguished, providing high sensitivity to the properties of each of the domains. Taking NaCl(001) as an example, we observe a discrete tilt angle distribution of the mosaic domains following an arithmetic progression with a 0.025° ± 0.005° difference; a twist mosaic angle of 0.09° ± 0.01° is also observed.

Grazing incidence diffraction of keV helium atoms on a Ag(110) surface.

Diffraction of fast atoms at grazing incidence has been recently demonstrated on the surface of alkali halides and wide band gap semiconductors, opening applications for the online monitoring of surface processes such as growth of ultrathin layers. This Letter reports energy resolved diffraction of helium on Ag(110) metal surface showing that a band gap is not mandatory to restrict the decoherence due to electron-hole pair excitations by the keV projectile. Measurement of the energy loss, which is in the eV range, sheds light on the scattering process.

Quantum scattering of fast atoms and molecules on surfaces.

We present evidence for the diffraction of light keV atoms and molecules grazingly scattered on LiF(001) and NaCl(001) surfaces. At such energies, the de Broglie wavelength is 2 orders of magnitude smaller that the mean thermal atomic displacement in the crystal. Thus, no coherent scattering was expected and interaction of keV atoms with surfaces is routinely treated with classical mechanics. We show here that well-defined diffraction patterns can be observed indicating that, for grazing scattering, the pertinent wavelength is that associated with the slow motion perpendicular to the surface. The experimental data are well reproduced by an ab initio calculation.

Exploration of small fibers for testing diabetic neuropathies.

INTRODUCTION: Electrophysiological exploration of neuropathies is a standard method of investigating the dysfunction of myelinated larger fibers (Aalpha, Abeta). However, this method cannot test dysfunctions in other fibers. To evaluate the smaller (Adelta) and unmyelinated fiber (C-fibers) lesions a quantitative method has been perfected: the study of the sensory thresholds (quantitative sensory testing: QST). It allows the investigation of the sensory symptoms and is a reproducible, non-invasive and painless method. It is used above all in patients suffering from diabetic neuropathy ('Diabetes Care 9 (1987) 432'). PATIENTS AND METHODS: We used the QST testing in comparison with nerve conduction velocities in 40 Non-Insulin-Dependent Diabetes Mellitus (NIDDM or Type II) patients in their 60s (+/-10 years). Depending on the duration of their diabetes (dd), we distinguished three groups: dd < 5 years (GI) dd from 5 to 15 years (GII) and dd > 15 years (GIII). All the patients underwent a clinical neurological examination, which enabled us to establish a gravity score comparable to the NDS (Neuropathy Disability Score: 'Muscle Nerve 10 (1988) 21'). RESULTS: Nerve conduction velocities and QST were studied for each group of patients. Electrophysiological alterations were connected to the gravity clinical score and in some asymptomatic patients a higher QST heat threshold could be observed. DISCUSSION: These results indicate that QST can detect the early dysfunction of the unmyelinated fibers in this kind of neuropathy. Subclinical detection can reduce severe neurological complications and make possible an early and effective treatment.

Evidence for F(-) formation by simultaneous double-electron capture during scattering of F(+) from a LiF(001) surface.

Slow F(+) ions (v<0.1 a.u.) scattered from a clean and flat LiF(001) surface under a grazing angle of incidence exhibit a high probability for forming F(-) ions in the reflected beam, whereas no negative ions are found for neutral F(0) projectiles. From detailed studies of projectile energy loss and charge transfer, we find evidence for a correlated double-electron capture process in the formation of the F(-) ions.

Electron bihole complex formation in neutralization of Ne+ on LiF(001).

Neutralization of low keV Ne+ ions at a LiF(001) surface is studied in a grazing incidence geometry. The combination of energy loss and electron spectroscopy in coincidence reveals two neutralization channels of comparable importance. Besides the Auger process, the Ne+ neutralization can proceed via peculiar target excitation, corresponding to the formation of an electron bihole complex termed trion.

Subsurface-channeling-like energy loss structure of the skipping motion on an ionic crystal.

The skipping motion of Ne+ ions in grazing scattering from the LiF(001) surface is studied for velocity below 0.1 a.u. with a time-of-flight technique. It is demonstrated that suppression of electronic excitation and dominance of optical phonon excitation in the projectile stopping results in an odd 1,3,5,... progression of the energy loss peaks, a feature usually ascribed to subsurface channeling. The experimental findings are well reproduced by parameter-free model calculations where thermal vibrations are the dominant cause for the ion trapping and detrapping.