An ellipsoidal mirror for focusing neutral atomic and molecular beams

Manipulation of atomic and molecular beams is essential to atom optics applications including atom lasers, atom lithography, atom interferometry and neutral atom microscopy. The manipulation of charge-neutral beams of limited polarizability, spin or excitation states remains problematic, but may be overcome by the development of novel diffractive or reflective optical elements. In this paper, we present the first experimental demonstration of atom focusing using an ellipsoidal mirror. The ellipsoidal mirror enables stigmatic off-axis focusing for the first time and we demonstrate focusing of a beam of neutral, ground-state helium atoms down to an approximately circular spot, (26.8±0.5) μm×(31.4±0.8) μm in size. The spot area is two orders of magnitude smaller than previous reflective focusing of atomic beams and is a critical milestone towards the construction of a high-intensity scanning helium microscope

An ellipsoidal mirror for focusing neutral atomic and molecular beams

Manipulation of atomic and molecular beams is essential to atom optics applications including atom lasers, atom lithography, atom interferometry and neutral atom microscopy. The manipulation of charge-neutral beams of limited polarizability, spin or excitation states remains problematic, but may be overcome by the development of novel diffractive or reflective optical elements. In this paper, we present the first experimental demonstration of atom focusing using an ellipsoidal mirror. The ellipsoidal mirror enables stigmatic off-axis focusing for the first time and we demonstrate focusing of a beam of neutral, ground-state helium atoms down to an approximately circular spot, (26.8±0.5) μm×(31.4±0.8) μm in size. The spot area is two orders of magnitude smaller than previous reflective focusing of atomic beams and is a critical milestone towards the construction of a high-intensity scanning helium microscope

The Structure of the α-quartz (0001) Surface Investigated Using Helium Atom Scattering and Atomic Force Microscopy

International audienceIn this paper we present the first structural investigation of quartz using helium atom scattering (HAS). Studies were carried out on the alpha-quartz (0001) surface (Z-cut). He-diffraction experiments reveal a complex hexagonal diffraction pattern. Prominent peaks are visible at positions corresponding to the periodicity of the bulk unit cell (4.90±0.02 °A). In addition weak (2×2) peaks and satellite peaks are observed. Two recent theoretical papers both propose a (1 × 1) reconstruction for alpha-quartz (0001) with 120° symmetry for the surface unit cell: the so called dense structure. We propose a model where terraces with step heights of a multiple 1/3 of the unit cell height lead to domains rotated by ±60° relative to each other. This is supported by AFM images. The satellite peaks indicate an incommensurate super-lattice structure with a periodicity of the order of 57 °A, presumably related to strain relief mechanisms. The (2 × 2) peaks may stem from co-existing regions of a semi-dense structure, which is predicted to have a higher energy than the dense, (1 × 1) phase

Vibrational Excitations of Glass Observed by Helium Atom Scattering

International audienceIn this paper we demonstrate a new application of helium atom scattering: the investigation of amorphous solids. Recently, we observed an excess density of states in the vibrational spectrum of the vitreous silica surface using inelastic He-atom scattering. A surface phonon spectral density was readily extracted from the recorded data and the excess density of states was attributed to the surface manifestation of the boson peak phenomenon of disordered structures. Here we present further data analysis and show that the intensity of the surface boson peak at constant energy, i.e., ½zz(¢E = const.,¢K)/½Debye, strongly depends upon the parallel momentum transfer with a maximum in the neighborhood of ¼ 1 nm−1. In contrast, the position of the maximum intensity of the surface boson peak shows negligible ¢K dependence and is seen as a dispersionless mode at 3.7±0.4 meV. Measurements of the width of the surface boson peak are also presented

Low-Energy Surface-Phonons on alpha-Quartz (0001)

International audienceCalculated surface-phonon dispersion curves for a dry, a partly hydroxylated, and a fully hydroxylated alpha-quartz (0001) surface are presented and compared with first low-energy inelastichelium-atom scattering measurements. Qualitative agreement between calculations and the observed Rayleigh-wave dispersion curve is achieved for the dry quartz surface. A signature effect is seen in the calculated phonon dispersion curves for the fully hydroxylated surface

Surface Dynamics Measurements of Silica Glass

International audienceWe present helium atom scattering experiments from the vitreous silica surface. Time-of-flight data of scattered He-atoms have been recorded at varying temperature and as a function of parallel momentum transfer. A dominant elastic peak and two broad features at ¼ 4 meV, one each on the phonon annihilation and creation side, are observed in all spectra. The thermal measurements between 127.0 and 368.5 K unveil clear Debye-Waller attenuation behavior of the elastic peak and indicate that scattering occurs in the single-phonon regime below 200 K. The root mean square vibration amplitude at room temperature is estimated to be 0.014 nm with a corresponding surface Debye temperature of 340 ± 20 K and an effective mass of 18 a.u. The obtained values agree well with bulk values under the assumption of force constants at the surface reduced by one-half. The wavevector dependence of the Debye-Waller exponent indicates a well depth of the atomsurface interaction potential of 10 ± 2 meV. The broad inelastic features on the phonon creation and annihilation sides are identified as the surface manifestation of the boson peak phenomenon in amorphous materials. An unusual shifting of the boson peak modes to higher energies with increasing temperature is observed. Measurements at different parallel momentum transfers unveil a non-dispersive behavior of the boson peak mode