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The singlet (o, ) and triplet (a&) parts of the total ionization cross section were determined in the energy range from threshold to 6ve times the threshold value with a crossed-beam arrangement of polarized electrons and spin-polarized He(23S) atoms. The measured spin asymmetry, As (cr, -cr&)/(o, +3o&), is compared with results for the lithium atom. Both asymmetries are quite similar. Comparison with a distorted-wave Born approximation shows strong disagreement in the vicinity of the threshold.The spin dependence of integrated electron-impact ionization cross sections has been studied experimentally in the energy range from threshold (Es) to about ten times the threshold value. Up to now diN'erent groups investigated the following atoms: H, ' Li, ' Na, and K. ' The results show that spin eff'ects are sizable and that the triplet contribution is suppressed but is by no means negligible. The value of the asymmetry at threshold, especially that obtained from an experiment with high-energy resolution, has provided clear evidence that at threshold and close to its higher partial waves, in addition to the S, wave, determine the cross section. This new insight gave rise to several theoretical treatments of the spin and orbital angular momentum behavior near threshold." Predicted asymmetry oscillations near threshold' have not been found experimentally.Unpolarized triple differential measurements on He ground-state atoms near threshold' were analyzed for their singlet and triplet partial wave content up to waves with L 2. ' ' The analysis is based on models of threshold behavior and yields information on triplet contributions for the special geometric configuration of oppositely escaping electrons. No triplet contribution is expected for events where the two electrons move out symmetrically.An ab initio theoretical treatment of the ionization process at low energies is a formidable problem. Progress in this field can only be made by extensive interaction between theory and experiment. In order to investigate the two-electron atom He, we looked at spin eA'ects in ionization of the metastable triplet state (2 S) for which the atomic electrons, the valence electron as well as the core electron, can be polarized to a high degree. The electronimpact ionization of atoms is a process whose energy dependence near threshold is dominated by the threeparticle Coulomb interaction in the outgoing channel, the energy variation of the electron-atom interaction in the incoming channel being less important. The threshold ionization of metastable helium is related to that of groundstate helium for incident electrons of correspondingly larger energy which lead to the same excess energy in the two-electron escape. Our polarization experiment with He(2 S) can be viewed as a way to study the ionization process for outgoing triplet two-electron wave functions which in the case of electron impact on ground-state heli-um atoms cannot be prepared. He+ ions were observed as a measure of the ionization rates Nt ) and Nt 1 for antiparallel an...

This beam was developed as a target for a crossed-beam electron-atom scattering experiment on the interaction of a polarized spin-½ electron with a polarized spin-1 atom. In the future this beam will be used in "Spin-Polarized Metastable Atom Deexcitation Spectroscopy" (SPMDS) for studying ferromagnetic surfaces without and with adsorbate layers. We use a discharge source for producing a beam of metastable helium atoms, a permanent sextupole magnet with a central stop at its exit for selecting He(23S) atoms in the Zeeman substate ms= + 1, a zero-field spin flipper for reversing the atomic beam polarization with respect to a magnetic guiding field, and a Stern-Gerlach magnet for analyzing the atomic polarization. At a distance of 90 cm beyond the exit of the sextupole, in the "interaction region" of an experiment, the polarized beam has a circular cross section of about 6 mm FWHM and a particle density of 1.10 v atoms/cm 3. The reversible spin polarization was determined as P=0.90+0.02. A possible contamination of the beam with metastable singlet atoms is included within this value; the ground-state He atoms are not considered to be part of the polarized beam. An observed contamination with long-lived Rydberg atoms can easily be destroyed by applying a high electric field.

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