Up to now, all triple-differential cross-section measurements were performed with unpolarized electrons and atoms and, consequently, averaged over direct and exchange scattering. In our crossed-beam experiment, spin-polarized lithium atoms were ionized by spin-polarized electrons of 54.4 eV energy. In coplanar, asymmetric coincidence detection of scattered and ejected electrons, combined with electronenergy analysis, we measured a spin asymmetry which gives new information about the ionization process. Results are compared with recent calculations based on the distorted-wave Born approximation.PACS numbers: 34.80.Dp, 34.8aNzThe triple-differential cross section (TDCS), d^a/dClA y^dflsdEA, of electron-impact ionization of atoms depends on the angles of the outgoing electrons A and B and on the partition of the energy between them, described by EA andwhere Eo is the incident electron energy and Ejon the ionization energy for the ejected atomic electron. By adopting nuclear-physics terminology, this process is usually called {e,2e) scattering.Most {e,2e) experiments belong to one of two categories. (1) (e,2e) spectroscopy, typically performed with noncoplanar, symmetric ±45° detector geometry and energies in the keV range for which the first-order theory is expected to be valid. Here the goal is to measure the momentum distributions of the bound electrons in atoms and molecules [1]. (2) Ehrhardt-type experiments, usually performed with asymmetric geometry at lower energies and with simple target atoms such as H, He, or Li. Here the goal is to test theories of electron-impact ionization [2]. Such tests have shown that first-order approximations suffice at high energies and manageable secondorder approximations are satisfactory at intermediate energies. Therefore, the current experimental efforts are concentrated at low energies where exchange effects are known to be important. Results from this laboratory on the total ionization of lithium and other alkali atoms near threshold show pronounced effects of exchange scattering [3]. This is the reason we developed an (e,2e) experiment with spin-polarized electrons and spin-polarized ^Li atoms. We chose lithium because it is-aside from the experimentally more demanding atomic hydrogen-the simplest one-electron atom. The isotope ^Li is used for technical reasons: It has a small hfs splitting of the ground state which insures an efficient high-field state selection in the hexapole magnet.

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