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Europhysics Letters (EPL)

Endothermal fine-structure transition of metastable argon atoms passing through a micro-slit copper grating

J.‐C. Karam, M. Boustimi, J. Baudon, M. Ducloy, F Perales, J. Reinhardt, Valerij Bočvarski, Constantin Maïnos, F Levron, Jacques Robert

Published Oct 1, 2002• 6 citations

Abstract

The endothermal fine-structure transition ( 3 P 2 → 3 P 0 —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.

DOI: 10.1209/epl/i2002-00340-1

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The endothermal fine-structure transition ( 3 P 2 → 3 P 0 —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.

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 |Δ m | −1/2 , where Δ m is the magnetic number change.

Absolute velocity-resolved total integral cross sections have been measured for collisions of helium singlet (2 1 S 0 ) and triplet (2 3 S 1 2) metastable (He*) atoms with ground-state helium, neon, and argon atoms in the thermal velocity range of 1.0-3.5*10 5 cm s -1 . Additional measurements on the He*-Ne system with a large input aperture at the detector to suppress the sensitivity of the apparatus to elastic scattering failed to show the previously predicted (Arathoon) sharply rising velocity structure in the inelastic excitation transfer cross sections.

Metastable argon atoms Ar*(3P2), produced by electron bombardment of a nozzle beam of ground state Ar atoms, are slowed down from their initial thermal velocity of 560 m/s down to a few tens of m/s, using a standard Zeeman slower. In this decelerator, a repulsive force is induced by a counter-propagating σ+- polarized laser beam, locked in frequency on the3P2-3D3closed transition (λ= 811.5 nm) and then detuned by 340 MHz. A special profile of longitudinal magnetic field is adjusted in order to maintain atoms in resonance with light all over the device, by compensating the variable Doppler shift by the convenient Zeeman shift. Low velocities, e.g. 55 m/s, are accessible but due to spontaneous emission randomly distributed recoil momentums enlarge both angular and velocity distributions of the beam. Nevertheless, by placing off axis collimating slits and grating, it is in principle possible to observe transmission and/or diffraction phenomena specific of low velocities.

van der Waals interactions between an atom and a planar surface exhibit a quadrupolar component in D(2)(z)-D(2)/3 (D, atomic dipole; ź, normal to surface). This coupling is responsible for an atom symmetry break, mixing levels of the same parity such as metastable 3P0, 3P2 levels of rare gas atoms. The strongly exoenergic 3P0-3P2 transition in Ar and Kr is observed by a time-of-flight technique, using as a surface the edge of a copper slit. The results confirm the predicted strong peaking of the angular distribution of inelastically scattered atoms and give a good estimate of the transition probability.

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 C 3 = 1.83 +0.1 −0.15 au being derived from spectroscopic data—leads to a good agreement with experiment.

The collision-induced exothermic fine-structure transition $\mathrm{Ne}(^{3}P_{0})\ensuremath{-}\mathrm{Ne}(^{3}P_{2})$ has been observed in a crossed-effusive-beam experiment, by measuring the time of flight of metastable atoms scattered perpendicularly to the incident-beam plane. In this direction, the elastic scattering is kinematically discriminated. The identification of the transition is achieved by pumping

^{3}P_{0}

or

^{3}P_{2}

levels with a cw dye laser.