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Journal of Applied Physics

Metastable atoms in a Mg beam: Excitation dynamics and velocity distribution

Giovanni Giusfredi, A. Godone, E. Bava, Carlo Novero

Published Mar 1, 1988• 20 citations
Publisher

Abstract

We describe the realization of a source of Mg atoms in the metastable triplet 3s3p3P and we report a theoretical model describing the dynamic process of production of metastable atoms via electron impact excitation. Experimental results concerning atomic flux and velocity distribution are reported and compared with the theoretical model; the efficiency of production of metastable atoms was 40%, in good agreement with the computed value, and the velocity distribution showed a dependence from the discharge current close to the theoretical prediction.

DOI
10.1063/1.339952

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We describe the realization of a source of Mg atoms in the metastable triplet 3s3p3P and we report a theoretical model describing the dynamic process of production of metastable atoms via electron impact excitation. Experimental results concerning atomic flux and velocity distribution are reported and compared with the theoretical model; the efficiency of production of metastable atoms was 40%, in good agreement with the computed value, and the velocity distribution showed a dependence from the discharge current close to the theoretical prediction.
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The excitation of a low-energy ground state helium beam to its metastable states by electron impact is analysed. It is shown that the narrowest velocity distribution for the resulting metastable helium beam is obtained by directing the helium and electron beams parallel or antiparallel. A source of metastable helium atoms was constructed according to this analysis which produced a beam of 3*10 14 metastable atoms/s sr with a velocity resolution of 3% FWHM for a room-temperature beam (E=66 meV). A possible source of errors in time-of-flight experiments with metastable atoms is pointed out.
For pt.I see ibid., vol.18, no.18, p.3557-37 (1985). A quantitative description of H(3s) atoms produced by laser excitation of a metastable hydrogen beam in an electric field is performed and compared with the recent experimental results of Claeys et al. (1985). The Stark effect for n=3 is calculated to obtain the relative intensities of the five Stark transitions as a function of the field strength. Good agreement with the experimental values is found. The dynamical behaviour of the 3s and 3p Stark states when the atoms leave the field is then analysed. Due to the adiabatic evolution we found that only one Stark state can produce H(3s) atoms.
A molecular beam source for high-energy metastable atoms, utilizing a corona discharge, is described. It was designed for scattering experiments where accurate alignment of beam components is required. The metastable atoms produced in this source have kinetic energies from 5 to 74 eV dependent on the species and sufficient velocity selection to reveal structure in the differential cross sections. These high-energy metastables were previously reported as photons. When the molecular beam source was operated at 200–300 Torr, a supersonic beam of metastables was produced. An optimization of operating conditions produced a supersonic beam flux two orders of magnitude greater than previously reported.
The optical pumping of atoms is a central process in many applications of atomic physics, such as laser cooling and the precise determination of physical constants. Popular methods for producing metastable helium atoms in supersonic expansions are discharge and electron-impact excitation. However, both approaches yield helium atoms in two metastable states, 21S0{2}^{1}{S}_{0}21S0​ and 23S1{2}^{3}{S}_{1}23S1​, which limits the applications of these highly energetic species. This study presents an experimental scheme in which the 21S0{2}^{1}{S}_{0}21S0​ state population is depleted via excitation to the 41P1{4}^{1}{P}_{1}41P1​ state, using diode-laser radiation. The results suggest nearly 100% depletion efficiency at all beam velocities studied.