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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.

An atom interferometer using two Stern-Gerlach magnets as polariser and analyser is described. The interferometer was first operated with a thermal beam of ground state potassium atoms. In that case the beam splitters are two radiofrequency zones within a transverse homogeneous magnetic field building a coherent superposition of Zeeman states. Ramsey fringes are obtained by scanning the RF-frequency through the resonance profile. Pulsing the RF power allows to get the timeof-flight distribution of the beam. Atomic interference fringes ("Stern-Gerlach fringes") are observed when the magnitude of an inhomogeneous magnetic field located in between the beam splitters is scanned. When the inhomogeneous field is also pulsed, the scalar Bohm-Aharonov effect is observed. The use of metastable helium atoms (He * 2 3 S) in place of potassium atoms is proposed. In addition to the usual advantages provided by metastable atoms of high internal energy, this atom offers the specific advantage to behave as an ideal spin-one particle. This allows us to greatly simplify the polarisation scheme of the interferometer. Possible applications involving transversally inhomogeneous magnetic fields are given and limitations due to the finite source size and angular aperture are discussed. Ramsey interference pattern obtained by scanning the RF frequency around 2.307 MHz (resonance). The RF power is such that the atoms experience two subsequent π/2 -pulses. The absolute contrast of the fringes is about 30%.

Surface-induced transitions between Zeeman sub-levels of metastable neon atoms traversing a micro-slit copper grating are investigated. The quadrupolar part of the surface van der Waals interaction suddenly perturbs the evolution of the Zeeman sub-levels in a magnetic field B, giving rise to van der Waals -Zeeman transitions. Exothermal inelastic transitions are identified by the related deflection γ. In a diagram (γ, B 1/2 ) the location of the inelastic peaks is fitted by straight lines of slopes proportional to |∆m| −1/2 , where ∆m is the magnetic number change.

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The endothermal fine-structure transition ( 3 P2 → 3 P0 -175 meV) of metastable argon atoms traversing at a velocity of 1030 m/s a micro-slit copper grating the normal axis of which is set, in a first configuration, at 60 • with respect to the incident beam axis, is observed. The related differential cross-section (DCS) is sharply peaked at 60 • and its velocity dependence reveals the expected threshold at 900 m/s. In a second configuration, the grating is set at an angle of 43 • , allowing for a 7% transmission of the incident beam. Atoms elastically scattered by the corner edges of the grating slits are observed in the angular range 1 • -4 • and the elastic DCS is measured in relative values. In spite of such a widely unsuited geometry, the endothermal process still appears (with a magnitude 10 times smaller than before), because of the surface corrugation. The inelastic DCS is now peaked at 57.5 • with a width of about 6 • , as predicted from the corrugation angle distribution deduced from the inelastic DCS of metastable N * 2 molecules traversing the same grating.

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