TRIPLY DIFFERENTIAL (E,2E) CROSS-SECTIONS FOR HE FROM THRESHOLD TO 50EV ABOVE

Distorted wave calculations of the triply differential cross sections for electron-impact ionization of He are presented for electrons shari ng less-than-or-equal-to 50 eV excess energy (E(x)) and having the cop lanar, theta12 = pi geometry. The results are in good agreement with a bsolute experimental measurements of T. Rosel et al. [Phys. Rev. A 46 (1992) 2539-2552] and of A. J. Murray, M. B. Woolf, and F. H. Read [J. Phys. B 25 (1992) 3021-30361. For final state electrons sharing 2 eV excess energy equally, many-body perturbation theory corrections to th e authors' first order (e,2e) triply differential cross sections [Phys . Rev. A 45 (1992) 4588-4603] are also presented. These results indica te that singlet-triplet interchannel coupling effects in the final sta te are very significant; that initial state correlations are significa nt near the perpendicular plane, i.e., theta1 = theta2 = pi/2; and tha t the polarization effects we include are insignificant. Finally, comp arison of our first order results with the semiclassical quantal theor y results of D. S. F. Crothers [J. Phys. B 19 (1986) 463-483] shows ex cellent agreement for excess energies E(x) greater-than-or-equal-to 2 eV and for theta1 = theta2 = pi/2; for E(x) < 2 eV, the two results di verge as E(x) --> 0, indicating that experimental tests of the Wannier -Peterkop-Rau theory of three-body Coulomb breakup should be carried o ut in this energy region.