Fine Structure Effect in Electron Impact Ionization

Abstract

Spin resolved (e,2e) experiments provide an extremely sensitive test of theories of electron impact ionization and many-body Coulomb effects. We present here results for (e,2e) collisions with xenon which show the first experimental evidence of the fine structure effect in electron impact ionization, analogous to the well known effect in electron impact excitation of atoms by polarized electrons. Comparison with distorted-wave Born-approximation calculations shows the sensitivity of the results to details of the target atom wave functions as well as the treatment of relativistic effects. PACS numbers: 34.80.Dp, 34.80.Nzwhere s " J and s # J are, respectively, the ͑e, 2e͒ differential cross sections to the final ion state of angular momentum J for incident electrons with spin up and spin down perpendicular to the scattering plane. Experimentally, A J is determined from the relation

Ionization of Atoms with Spin Polarized Electrons

“…The model predicts zero spin asymmetries in the limit of negligible exchange, and in analogy to the "Fine-structure Effect" in excitation [10,11], zero spin asymmetry for the case where the individual finestructure transitions are energetically unresolved. Subsequent measurements on xenon [12,13] and later, more sophisticated calculations [14,15], while confirming a strong spin-dependence in the ionization cross sections as originally predicted, have revealed additional and significant contributions from many-body exchange effects which tend to mask the signatures for a "pure" fine-structure effect. Notwithstanding this fact, that [9] predicts zero spin asymmetry in the absence of exchange means that the spin asymmetry parameter is a very sensitive test of the role and nature of electron exchange in electron-impact-induced ionizing collisions, and a sensitive test to the magnitude of contributions from relativistic effects which can also contribute to a nonzero result, even in the absence of exchange.…”

“…Xenon was the first closed-shell target atom for which the measurement of nonzero spin asymmetries was established [12,13]. Since that time disparities between experiment and theory have narrowed, leading to an enhanced understanding of scattering dynamics.…”

Invited Article: An improved double-toroidal spectrometer for gas phase (e,2e) studies

“…The reason for the lower than expected value of polarization is presently unclear; however, it still represents a considerable improvement over the 24% reported in our earlier measurements. 18 After extraction, the spin-polarized photoelectrons are deflected through 90°by an electrostatic field to produce a beam of transversely polarized electrons. Transverse polarization is a necessary condition 18 to observe nonzero spin effects in the present reaction geometry in which the momentum vectors for the incident and detected scattered electrons are confined to a common plane ͑the so-called scattering plane͒.…”

“…18 After extraction, the spin-polarized photoelectrons are deflected through 90°by an electrostatic field to produce a beam of transversely polarized electrons. Transverse polarization is a necessary condition 18 to observe nonzero spin effects in the present reaction geometry in which the momentum vectors for the incident and detected scattered electrons are confined to a common plane ͑the so-called scattering plane͒. The electron beam is then accelerated to around 1 keV of energy and transported at high energy through a differential-pumping stage before entering the collision chamber in which the toroidal electron analyzers are housed.…”

“…In addition, inherent to the technique are significant nonuniformities in momentum resolution over the measured range of momentum phase space. While these are not serious drawbacks in many experiments, they do pose problems in others, for example, experiments where the dynamics of the collision process depends strongly on the angular-momentum state of the final ion which is determined by resolving closely spaced fine-structure levels, 18 or in measurements where resolution of sharp angular structures is crucial to probing relativistic effects, such as for the scattering of electrons from heavy targets. 19 Moreover, high values of energy resolution and uniform angular resolution are desirable characteristics for so-called electron momentum spectroscopy measurements.…”