Spin-Orbit Interaction of the Continuum Electrons in Relativistic (e, 2e) Measurements

Abstract

Using a transversely polarized electron beam a relativistic (e, 2e) experiment has been performed to look for a spin up-down asymmetry in the electron-impact ionization process caused by the spinorbit interaction of the continuum electrons in the Coulomb field of the atomic nucleus. An incident energy of 300 keV, coplanar asymmetric kinematics, and the It shell of silver (Z = 47) have been used. We found a distinct spin asymmetry in the recoil peak (up to 16'), whereas in the binary peak the asymmetry is close to zero. This feature is confirmed by theoretical calculations, but quantitatively the agreement is poor. PACS numbers: 34.80.Dp, 34.80.Nz The study of the dynamics of electron-impact ionization of atoms by electron-electron coincidence [or (e, 2e)] experiments has proved to be a valuable method of investigating the physics of ionization processes [1,2]. The application of spin-polarized electron beams allows for an even more severe test of theory, and recently results have been reported for low energies. In these nonrelativistic (e, 2e) experiments spin-dependent asymmetries have been studied which are due to the exchange interaction [3] and to the so-called fine-structure effect [4,5].In this Letter we report on a relativistic (e, 2e) experiment with a transversely polarized electron beam designed to look for a spin asymmetry caused by the spin-orbit interaction of the continuum electrons in the Coulomb field of the atomic nucleus. The spin-orbit interaction arises from the interaction of the magnetic moment of the electrons with the magnetic field felt in the rest frame of the electrons because of their motion in the Coulomb field of the target nucleus (Mott scattering). As a result, a spin up-down asymmetry is to be expected in the triply differential cross section of electron-impact ionization. In particular, we were interested in investigating the angular distribution in view of the question whether the asymmetry is larger within the region of the so-called recoil peak than the asymmetry within the binary peak. Our assumption is based on the following intuitive argument. As the binary peak has a large contribution from a direct binary collision between the incoming electron and the atomic electron with the nucleus in the role of a "spectator, " the spin-orbit interaction will be weak. The recoil peak, however, cannot be explained unless an electron-nucleus interaction is taken into account. Consequently, here a spin-orbit interaction must contribute, and a spin asymmetry is to be expected.A sketch of the experimental arrangement is shown in Fig. 1. The source for the polarized electron beam (described in detail elsewhere [6]) used the photoemission of electrons from a GaAsP crystal irradiated by circularly polarized light of a helium-neon laser. After being deflected by a 90 cylindrical deflector, the extracted electrons are transversely polarized. The spin Hip of the electron beam can be easily realized by reversing the helicity of the laser light. The source is installed in a high voltage termina...

Fine Structure Effect in Electron Impact Ionization

“…Performing their experiment with an extremely light target they were able to investigate the scattering process under conditions where spin-orbit inter-action of the continuum electrons in the atomic and ionic fields is negligible. In contrast, the Tübingen group [10] recently measured spin asymmetries in the ͑e, 2e͒ scattering cross section for the ionization of K-shell electrons in unpolarized silver atoms by high energy 300 keV polarized electrons. In this case, the spin asymmetries result from the spin-orbit interaction of the continuum electrons.…”

ESR Investigation of a Spin 1/2 Chlorine-Associated Defect in Synthetic FeS2 Crystals

“…It was argued by Prinz et al (1995) and predicted by Keller et al (1996) that asymmetries induced by continuum spin-orbit coupling should be strongly Z dependent. Using a transversely polarized electron beam of 300 keV we have, therefore, measured the spin asymmetries of the triply differential cross sections for electron-impact ionization of the K-shells of copper, silver and gold.…”

“…Recently, two types of relativistic (e, 2e) experiments using transversely polarized electron beams and unpolarized targets have been carried out to measure spin up-down asymmetries. In the first type, spin asymmetries have been measured on the K-shell of silver (Prinz et al 1995) and on the L-shell of gold (Prinz and Keller 1996). These asymmetries are caused by spindependent forces, i.e.…”

“…These asymmetries are caused by spindependent forces, i.e. by spin-orbit interactions of the continuum electrons moving with relativistic energies in the Coulomb field of the atomic nucleus (Prinz et al 1995, Prinz and. In an experiment of the second type on the 2p 3/2 subshell of uranium (Besch et al 1998) the spin asymmetries are interpreted in terms of the so-called fine-structure effect also known from the nonrelativistic energy regime (see, e.g., Dorn et al 1997, Jones et al 1994.…”

Spin Effects in the (e, 2e) Cross Section of Xenon

“…In this case the summations in equation (1) over the spin direction of the incident electron and the magnetic states of the target atom disappear, significantly reducing the number of spin-dependent reaction channels over which the experiment is averaging. Such experiments are still in their infancy (Baum et al 1992;Prinz et al 1995;Granitza et al 1993;Simon et al 1993). Elimination of the summation over Vs and Vf (for M j non-zero) requires the employment of spin-sensitive detectors for the two final state continuum electrons as well as determination of the angular momentum projection of the residual ion.…”