Energy transfer in hyperthermal Xe-graphite surface scattering

The scattering of a hyperthermal Xe from a graphite (0001) surface has been studied using a molecular beam-surface scattering technique and molecular dynamics (MD) simulations. The angular and velocity distributions of scattered Xe atoms were measured at incidence energies from 0.45 to 3.5 eV, three incidence angles of 15°, 35° and 60° and the surface temperatures of 300 K and 550 K. The observed time-of-flight spectra exhibit a sharp velocity distribution with only one velocity component, which is ascribed to the direct inelastic scattering process. The angle-resolved energy ratios of the mean final translational energy over the mean incidence energy E_f/E_i agree well with those predicted based on the assumption of the conservation of the momentum parallel to the surface. The Hard-Cube model, where the mass of the cube is approximately 310 u, has reproduced the angle-resolved flux distributions of scattered Xe atoms. In the Hard-Cube model almost 80% of the normal component of the incidence translational energy is found to be lost in collision. The MD simulations reproduce well the experimental results by using the Brenner potential for intralayer C atoms and a Lennard-Jones potential for interlayer C–C pair interactions.     

Stern Gerlach interferometry with metastable argon atoms: an immaterial mask modulating the profile of a supersonic beam

A new Stern Gerlach interferometer operating with a nozzle beam of metastable argon atoms Ar^* (3p⁵ 4s, ³ P ₂) is described. The selection of incoming (polarisation) and outgoing (analysis) Zeeman sublevels is achieved by use of laser induced transitions at two wavelengths, 811.5 nm (closed J = 2 → J = 3 transition) and 801.5 nm (open J = 2 → J = 2 transition). Linear superpositions of Zeeman sublevels, just beyond the polariser and just before the analyser, are prepared by means of two zones where Majorana transitions take place. In between, a controlled magnetic field configuration (the phase object) is produced within a triple μ-metal shielding. Standard interference patterns are obtained by scanning the field and detecting the atoms by secondary electron emission from a Faraday cup. When a static radial magnetic gradient is used, the beam profile is modulated by interference. The transverse pattern, which can be translated at will by adding a homogeneous field, is observed for the first time using a multi-channel electron multiplier followed by a phosphor screen and a CCD camera. The results satisfactorily agree with all theoretical predictions. Received 27 June 2002 / Received in final form 20 September 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: perales@lpl.univ-paris13.fr RID="b" ID="b"UMR 7538 du CNRS     

Modelling the diffraction of laser-cooled atoms from magnetic gratings

The diffraction of laser-cooled atoms from a spatially-periodic potential is modelled using rigorous coupled-wave analysis. This numerical technique, normally applied to light-diffraction, is adapted for use with atomic de Broglie waves incident on a reflecting diffraction grating. The technique approximates the potential by a large number of constant layers and successively solves the complex eigenvalue problem in each layer, propagating the solution up to the surface of the grating. The method enables the diffraction efficiencies to be calculated for any periodic potential. The results from the numerical model are compared with the thin phase-grating approximation formulae for evanescent light-wave diffraction gratings and idealised magnetic diffraction gratings. The model is applied to the problem of diffracting Rb atoms from a grating made from an array of permanent magnets. Received 13 June 2000 and Received in final form 15 December 2000     

Investigations of “soft-landed” Cd surface atoms via nuclear methods: hyperfine-field sign determination

Using refined preparation techniques, cadmium guest atoms have been positioned at different sites on the surfaces of nickel crystals. The magnetic hyperfine fields and the electric field gradients at the Cd nuclei were measured by time-dependent perturbed angular correlation (TDPAC) spectroscopy of the emitted gamma radiations. By measuring the combined interactions, electric field gradients and magnetic hyperfine fields can be unambiguously attributed to each surface site. The signs of the magnetic hyperfine fields are determined by applying an external magnetic field and choosing the appropriate γ-ray detector configuration. The measured fields correlate with the number of neighbouring host atoms. Band structure calculations confirm this finding and predict magnetic fields for various sp elements from the band structure of the s-like conduction electrons. The quadrupolar interactions are manifestations of the balance in the occupation of the guest p-sublevels. These results provide new information on the structure and formation of electronic configurations of sp elements in different local environments and will contribute to understanding electronic effects on surfaces.     

Experiments on the reflection of cold atoms from magnetic thin films

We report the results from a series of experiments in which ferromagnetic thin films were used as atom mirrors for laser-cooled rubidium atoms released from a magneto-optical trap. The thin films were made of cobalt and lanthanum calcium manganite (LCMO) with thicknesses between 20 and 300 nm. The magnetic domains in these thin films have a periodic structure where the spatial period is of the order of the thickness of the film, and the field decays exponentially above the film over a length scale comparable to the domain size. Thus, the neutral atoms reflect off these films from distances comparable to the thickness of the film, resulting in modification of the reflectivity due to the competition between the repulsive magnetic force and the attractive short-range forces such as van der Waals and Casimir forces. The smoothness of the atom mirror is also modified due to the proximity of the magnetic domains. The reflectivity is sensitive to the domain structure and size, which can be modified in LCMO by applying a modest external magnetic field. In this paper, we discuss the evaluation of the thin films as magnetic mirrors for atom optics, and the measurement of the van der Waals force with an accuracy of about 15%, using cobalt thin films. We also discuss some preliminary results on the temperature-dependent reflectivity for atoms near the ferromagnetic transition at 250 K in the LCMO film, and on the domain dynamics and relaxation.     

Metastable rare gas atoms scattered by nano- and micro-slit transmission gratings

The transmission of metastable argon atoms through nano-slit or micro-slit gratings is studied by use of time of flight and angular analysis. This transmission departs from the simple geometric one essentially by two ways: (i) the elastic or diagonal part of the van der Waals (vW) interaction with the solid causes an angular narrowing of the emerging beam; (ii) the off-diagonal vW interaction induces the exothermal fine structure transition ³ P ₀↦³ P ₂ (ΔE = 175 meV) leading to large scattering angles; the resulting angular distribution is very sensitive to the roughness of the surface in the direction of the depth. An extension of these experiments to transversally coherent beams is proposed. It should be considered as a first step towards a new type of interferometer in which the inelastic diffraction makes the gratings work as beam splitters or mirrors. Received 6 July 2001 and Received in final form 17 September 2001     

Magnetic interactions of cold atoms with anisotropic conductors

We analyze atom-surface magnetic interactions on atom chips where the magnetic trapping potentials are produced by current carrying wires made of electrically anisotropic materials. We discuss a theory for time dependent fluctuations of the magnetic potential, arising from thermal noise originating from the surface. It is shown that using materials with a large electrical anisotropy results in a considerable reduction of heating and decoherence rates of ultra-cold atoms trapped near the surface, of up to several orders of magnitude. The trap loss rate due to spin flips is expected to be significantly reduced upon cooling the surface to low temperatures. In addition, the electrical anisotropy significantly suppresses the amplitude of static spatial potential corrugations due to current scattering within imperfect wires. Also the shape of the corrugation pattern depends on the electrical anisotropy: the preferred angle of the scattered current wave fronts can be varied over a wide range. Materials, fabrication, and experimental issues are discussed, and specific candidate materials are suggested.     

Cooling and alignment of ethylene molecules in supersonic seeded expansions: diagnostic and application to gas phase and surface scattering experiments

Rotational cooling and collisional alignment of ethylene molecules is induced by seeding effects in supersonic expansions with lighter gas carriers such as He and Ne. The dependencies of the degree of alignment on the rotational state, on the final speed of the molecules and on the diffusion angular cone have been characterized by coupling two different experimental methodologies. An application to surface scattering is then demonstrated by measuring stereo-dynamical effect in the adsorption on metallic surfaces.     

Long-range atom-metal-surface interaction and interference of atomic states

The properties of two-dimensional magnetic traps for laser-cooled atoms are analysed using complex functions. The two components of the magnetic field from a series of parallel, infinitely long, current-carrying wires are represented by a single complex number. The regions of the field where paramagnetic atoms can be trapped occur where the magnetic field is zero. The locations of the zeroes of the field are obtained as the solution to a polynomial and the multiplicity m of the solution determines both the 2(m + 1)-pole nature of the trap and the field gradient through the centre. The zeroes of the field can be merged or split by varying the locations of the currents, their strengths or by applying a uniform magnetic field. The theory is applied to magnetic traps created from long thin wires or permanent magnets on a substrate. The properties of a number of magnetic trap configurations used for atom guides are discussed. Received 28 February 2001 and Received in final form 6 July 2001     

Magnetic reconstructions at the surface of the B2 FeV alloy

We present an ab initio study of the magnetic surface reconstructions of the B2 FeV alloy using a self-consistent tight-binding linearized muffin tin orbital method developed in the atomic spheres approximation. For (001) and (111), the surface reconstruction stabilizes configurations unstable in the bulk alloy. When Fe is at the (001) surface, a c(2×2) in-plane antiferromagnetic order is found to be the ground state with magnetic moments of -2.32 and 2.27. A p(1×1) ↓ ferromagnetic order is displayed in case of V toplayer with a magnetic moment of -1.83. At the (111) surface, we obtain for Fe toplayer two solutions p(1×1)↑ and p(2×1). The configuration p(1×1)↑ is found to be the ground state with a magnetic moment per atom of 2.34. For V toplayer, only the p(1×1) ↓ solution is obtained with a moment of -0.84. In all cases, the Fe-V coupling is always antiparallel like in the bulk. Our results are discussed and compared to experiments. Received 11 August 2000 and Received in final form 8 June 2001     

Rotational cooling efficiency upon molecular ionization: the case of Li2(a3Σu+) and Li2+(X2Σg+) interacting with 4He

The low-temperature (up to about 100 K) collisional (de)excitation cross sections are computed using the full coupled-channel (CC) quantum dynamics for both Li₂ and Li₂ ⁺ molecular targets in collision with ⁴He. The interaction forces are obtained from fairly accurate ab initio calculations and the corresponding pseudo-rates are also computed. The results show surprising similarities between sizes of inelastic flux distributions within final states in both systems and the findings are connected with the structural change in the molecular rotor features when the neutral species is replaced by its ionic counterpart.     

Fast measurement of rocking curve of reflection high-energy electron diffraction by using quasi-1D convergent beam

In order to investigate dynamic change in surface structure, we tried to measure the rocking curve of reflection high-energy electron diffraction (RHEED) by using a quasi-1D convergent beam method, which means a superposition of a hundred RHEED patterns at different glancing angles on one image. After we have checked the experimental accuracy, it is confirmed that the measurement time is shortened to 0.3 s by using this method. We also show an application of this method to a real time observation of surface segregation of B atoms while a highly B-doped Si is annealed.     

Surface structure analysis based on the exclusive use of the specular LEED spot – a theoretical study

The exclusive use of the specularly reflected beam (the (0,0) spot) may be a more practical way of collecting data for a LEED I–V structure analysis under certain experimental conditions. In this paper we discuss the special properties of the (0,0) spot intensity and test its sensitivity towards structural changes for the model system CO/Ni(1 1 1) within the framework of a R factor analysis. It is found that the (0,0) spot can, indeed, be used for a reliable structure determination if the energy range is increased by collecting data at different polar and azimuthal angles of incidence. The R factor contrast is, however, reduced with respect to a conventional LEED I–V analysis.     

Collision-energy-resolved Penning ionization electron spectroscopy of styrene, 2-vinylpyridine, and 4-vinylpyridine with He*(23S) metastable atoms

Collisional ionization of styrene (phenylethylene), 2-vinylpyridine, and 4-vinylpyridine with metastable He*(2³S) atoms were studied by means of collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectroscopy. Collision energy dependence of partial ionization cross-sections, which reflects the anisotropic interactions between a He*(2³S) atom and the target molecules, indicates that attractive interaction for the out-of-plane access of a He*(2³S) atom to phenyl group is stronger than that for the out-of-plane access to vinyl group. Moreover, it was found for vinylpyridines that the attractive interaction around π electrons became weaker than that for styrene, and that the attractive interaction for the in-plane access to the nitrogen atom is stronger than that for out-of-plane π-directions. However, in 2-vinylpyridine, the hydrogen atom of vinyl group prevents a He*(2³S) atom from approaching to the nitrogen atom along in-plane directions, and thus the attractive interactions around the nitrogen atom were shielded by the vinyl group. The experimentally observed anisotropic interactions were qualitatively supported with ab initio model interaction potential calculations between a Li (He*(2³S)) atom and the target molecule. Concerning with electronic structures of investigated molecules, the assignment of Penning ionization electron spectrum for 4-vinylpyridine was discussed on the basis of different behavior of collision-energy dependence of partial ionization cross-sections, and the satellite ionization band in Penning ionization electron spectra was also reported for styrene.     

The azimuthal dependent oxidation process on Cu(110) by energetic oxygen molecules

We report a study on the oxidation process induced by a hyperthermal oxygen molecular beam (HOMB) on Cu(110) using X-ray photoemission spectroscopy in conjunction with a synchrotron radiation source. The oxidation process induced by energetic O₂ beams on Cu(110), depending on the azimuthal angle of incidence, suggests that the –Cu–O– added row structure has a role in inhibiting adsorption as a steric obstacle for incident O₂ molecules.     

Substitutional adsorption of K on Fe(100)

The adsorption of potassium on Fe(100) was studied by time-of-flight forward scattering and recoiling spectroscopy (TOF-SARS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After heating to 650 K of the potassium saturated surface the formation of a p(3 × 3) potassium superstructure was observed by LEED. TOF-SARS experiments ruled out the adsorption of potassium in the on-top, bridge and four-fold hollow site. The only site which is in agreement with all experimental results is the substitutional site where K replaces an Fe atom of the topmost layer of the crystal. This is the first time a substitutional adsorption site has been found on a bcc surface. On an fcc surface such an adsorption site has been found recently for adsorption of sodium and potassium on Al(111).     

Investigation of phosphorus surface segregation by X-ray scattering measurements

The surface segregation of phosphorus in silicon at low temperatures is studied by using δ doping structures grown by molecular beam epitaxy. The samples are characterized by X-ray crystal truncation rod (CTR) scattering using synchrotron radiation as the light source. The 1/e decay length of P segregation and segregation barrier energy are obtained by fitting the CTR curves within kinematical approximation of X-ray diffraction theory. The surface segregation of P is strong at a growth temperature of 450 °C, with a 1/e decay length of 14 nm, while for growth temperatures below 350 °C, P segregation is negligible with a 1/e decay length not larger than 4 nm. The segregation barrier energy is determined to be 0.43 eV.     

Restoring entanglement in atomic collisions: A gedanken experiment

A new type of collision experiments is discussed, where observations of two successive collisions of the same pair of particles would be possible. When such technology is available, a surprising restoring of entanglement, normally considered broken in usual collision experiments, could be observed. As an illustration the collision partners He⁺ and He⁺⁺ in a collision regime where the resonant charge transfer is dominating are considered. In the analysis it is shown that in such experiments, two spatially widely separated ion paths, corresponding in fact to two different charge states, would contribute coherently to the final amplitudes, describing which of the ions emerges as singly charged, i.e. which carries the single electron involved. The double collision experiments are not trivial, since their overall cross-sections are extremely small. Development of relevant experimental techniques will decide if the proposed phenomena remain in the field of gedanken experiments or enter the world of real experimental physics. Received 2 December 1999 and Received in final form 12 May 2000