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.

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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communicated during the construction of the apparatus 8 and the experimental data is satisfactory. The differences might be due to uncertainties of the experimental resonance cross sections or to the possible existence of a 6 2 S 1/2 autoionization transition 9 near 1300 A. According to the measurement the value P = -50% [where Q s ' = 0, Eq. (4)] is at the wave number 64 400 ± 600 cm" 1 . Since from Eq. (2) e 0 = -# 0 for Q s ' = 0, Eq.(3) together with the measurement yields T 0 q 0 = 5500 ±1200 cm" 1 for the 6 2 P 1/2 autoionization transition.We have shown that polarized electrons can be obtained from autoionizing transitions and that the polarization can be utilized for the analysis of the autoionization resonances. It is worth noting that the intensity distribution 1(A) of the ionizing light (depending on the light source and the absorption coefficients of the optical devices) does not have to be known for the measurement of the wavelength-dependent P(X), since the electron polarization is obtained from the ratio of the electron intensities in the two counters of the Mott detector.The interpretation of excitation processes in ion-atom collisions at moderate energies (a few hundred eV) is based on a molecular description of the colliding-particle system. In most cases, the primary excitation mechanism consists in an interaction between ground and excited molecular states, which occurs at a rather small internuclear distance (a few bohrs). In general, many coherent scattering amplitudes, corresponding to several output channels, are produced at the same time by the collision and these amplitudes can interfere at large or infinite internuclear distances. Thus, in spite of the fact that the primary excitation mechanisms are now generally We gratefully acknowledge support by the Deutsche Forschungsgemeinschaft. 1 U. Fano, Phys. Rev. _178, 131 (1969).transitions from the 6 2 P 3 / 2 state can be neglected because this state is only populated with a probability of less than 3xio' 4 . 3 N.well understood, transition probabilities-or inelastic differential cross sections--for excitation to one specific atomic level are often difficult to determine because of these "secondary" interactions occurring at large distances. For the particular case of the excitation of a 1 P state of helium, in a He + -He collision, for a specific scattering angle (the ion is assumed to remain in the \S state), different and coherent amplitudes a m (6, <*>) are obtained for the different magnetic sublevels m = -1, 0, + 1, with the symmetry relation a i = -a~i ( see Macek and Jaecks 1 ), As the collision time at moderate energies (10~1 5 sec) is much shorter than the lifetime of the excited The polarization of photons from the transition 3 3 P -* 2 3 S and the angular distribution of radiation from the decay 2 l P~+ 1*5 emitted in He + +He collisions at an incident energy of 150 eV and an ion scattering angle of 13.5° were measured by detecting the scattered ions and photons in delayed coincidence. From these measurements we conclude that t...

In the present experiment, the motional electric field responsible for the quenching (and polarization) of metastable hydrogen atoms passing through a magnetic field, is compensated, for a prescribed atomic velocity, by a static electric field. The resulting continuous velocity selection has been tested over a wide range of velocities ((5 ÷ 40) km/s). The relative velocity dispersion ranges from 5% at 40 km/s, to 22% at 5 km/s. As is confirmed by a calculation, the resolution is mainly determined by the quality of the matching of the two field profiles.

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