V. Bocvarski

Atom diffraction with a ‘natural’ metastable atom nozzle beam

The resonant metastability-exchange process is used to obtain a metastable atom beam with intrinsic properties close to those of a ground-state atom nozzle beam (small angular aperture, narrow velocity distribution). The estimated effective source diameter (15 µm) is small enough to provide at a distance of 597 mm a transverse coherence radius of about 873 nm for argon, 1236 nm for neon and 1660 nm for helium. It is demonstrated both by experiment and numerical calculations with He*, Ne* and Ar* metastable atoms, that this beam gives rise to diffraction effects on the transmitted angular pattern of a silicon-nitride nano-slit grating (period 100 nm). Observed patterns are in good agreement with previous measurements with He* and Ne* metastable atoms. For argon, a calculation taking into account the angular aperture of the beam (0.35 mrad) and the effect of the van der Waals interaction—the van der Waals constant C3 = 1.83+0.1−0.15 au being derived from spectroscopic data—leads to a good agreement with experiment.

Study of low-energy resonant metastability exchange in argon by a pulsed merging beam technique

The resonant metastability exchange process in low-energy collinear collisions between metastable argon atoms (Ar* 3P2) polarized in spin (M = +2) and ground-state Ar atoms from a nozzle beam is studied by means of a time-of-flight technique. A wide range of metastable atom velocities in the laboratory frame (275 m s−1 down to 50 m s−1) is obtained by use of a Zeeman slower, the counter-propagating laser beam of which is locked in frequency onto the 3P2–3D3 closed transition (λ = 811.5 nm). The accessible centre-of-mass energy range (8–27 meV) has not been explored so far, to our knowledge. Calculations based upon existing interatomic potentials of 2g and 2u symmetries are in reasonable agreement with experiment.

High resolution electron imaging system for sub-micron sized metastable atom beams produced by Stern–Gerlach interferometry

The method of modulating an atom beam profile by an immaterial magnetic mask generated in a Stern–Gerlach interferometer is recalled. A special magnetic configuration aimed at producing a single central bright interference fringe (atomic spot) was used. The effects of velocity spread, source coherence and source size on the limiting spot size at large values of the magnetic gradient are discussed. The observation of such small sizes requires a high spatial resolution of the position-sensitive detector. A new electron optical device is described, which images the secondary electron source generated by the impact of the atomic beam on a metallic electrode (detection in real time). Magnifications as high as 65 are accessible, leading to a better than 100 nm resolution of the atomic beam profile when a position-sensitive detector of a few µm resolution is used. Geometric and chromatic aberrations are discussed and, according to simulations, they do not significantly deteriorate the resolution.

Abinitio calculation of adiabatic potential surfaces for Ne*(2p53s,1,3P)+H2(1Σ+g)

Adiabatic potential surfaces have been computed ab initio for molecular states dissociating into Ne* (2p 53s,1,3P)+H2(1Σ+g). The distance R between Ne* and H2 ranges from 5 up to 50 a0. The distance r between H atoms is kept constant, equal to its equilibrium value (1.4 a0). The interaction with the electronic continuum is ignored. Potentials, including spin–orbit interaction, are fitted to analytic forms. Specific molecular properties of the system are discussed.

Atom-surface interaction at the nanometre scale: van der Waals-Zeeman transitions in a magnetic field

van der Waals-Zeeman transitions between magnetic states of metastable rare-gas atoms Ar*, Kr* and Xe* (3P2) induced by a solid surface in the presence of a magnetic field, are investigated theoretically and experimentally. By use of a Zeeman slower, metastable argon atoms with various velocities ranging from 170 to 560 m/s allow us to investigate the small impact parameter range (3–7 nm) within which these transitions occur, as well as the effect of atom polarisation on the sharing out of the M states.

Coherent Atom Optics With Fast Metastable Beams: Metastable Helium Diffraction By 1D and 2D Magnetized Reflection Gratings

1D and 2D reflection gratings (Permalloy stripes or dots deposited on silicon), immersed in an external homogeneous static magnetic field, are used to study 1D and 2D diffraction of fast metastable helium atoms He* (23S1). Both the grazing incidence used here and the repulsive potential (for sub‐level m = −1) generated by the magnetisation reduce the quenching effect. This periodically structured potential is responsible for the diffraction in the incidence plane as well as for the diffraction in the perpendicular plane.

Surface-induced vibrational excitation of metastable nitrogen molecules traversing a micro-slit copper grating: a probe of surface profiles

The interaction at mean distance (a few tens up to a few hundreds of a0), i.e. in the van der Waals interaction range, between metastable nitrogen molecules, N2* (A3Σu+), and the slit edges of a micro-slit copper grating depends on both the molecular orientation and the internuclear distance in the molecule. Such an interaction is able to induce rotational and vibrational transitions. Endo-energetic transitions (v → v + 1, v ranging from 5 to 10) are observed by means of a time-of-flight technique combined with an angular distribution measurement. By setting the grating plane at an angle with respect to the incident direction, different from that imposed by ideally planar slit walls, it is shown that the angular distribution of the inelastic process reveals a departure of the surface from an ideal plane. Assuming a regular evolution of the tangent plane along the surface profile, a mean wall profile can be derived from this distribution.