First and second Born (e, 2e) calculations are presented for excitation-ionization of ground-state helium to He + (n = 2). Results for ionization to the ground-state ion He + (1s) are also given. The physical content of the approximations is discussed, in particular, the two-step mechanism which appears in the second-order term for excitation-ionization. The second Born term is calculated in the closure approximation using a new numerical method based on prolate spheroidal coordinates. Comparison is made with absolute experimental data from Paris and Rome in very asymmetric coplanar geometry-scattered electron energies of 5500, 1500 and 570 eV and ejected electron energies of 5, 10, 20, 40 and 75 eV. For excitation-ionization the second Born approximation generally gives improved agreement with the experimental data in the recoil region and second-order effects are found to be still significant at 5500 eV. The importance of the second-order term decreases with increasing ejected energy for the cases studied here. † Whereas the 2s → 1s transition is not 'optically allowed', the 2s state can decay radiatively to 1s by emission of two photons. These photons will have a distribution of energies and will therefore not appear as a single spectral line like the optically allowed 2p → 1s decay.‡ And also with collisions of other projectiles such as Li q+ , B q+ , C q+ , H + 2 and H +
We report detailed calculations of the first Born triple-differential cross section for (e, 2e) excitation-ionization of ground state He to He + (n = 2). These are in accord with the very recent work of Kheifets et al (1999 J. Phys. B: At. Mol. Opt. Phys. 32 L433) and confirm that the first Born amplitude is now known very accurately. We illustrate the sensitivity of the first Born cross section to the choice of initial and final state wavefunctions. We combine our accurate first Born amplitude with an estimate of the second Born term evaluated in the closure approximation. As in previous work (Marchalant et al 1998 J. Phys. B: At. Mol. Opt. Phys. 31 1141 we find that second-order effects are significant even up to energies as high as 5.5 keV. Agreement with experiment generally remains not very satisfactory.
We have performed a kinematically complete experiment and
calculations on single ionization in 100 MeV/amu C6+ + He
collisions. For electrons ejected into the
scattering plane (defined by the initial and final projectile
momentum vectors) our first- and higher-order calculations are
in good agreement with the data. In the plane perpendicular to
the scattering plane and containing the initial projectile axis
a strong forward-backward asymmetry is observed. In this plane
both the first-order and the higher-order calculations do not
provide good agreement neither with the data nor amongst each
other.
Simultaneous excitation-ionization of helium to the He + (2p) ionic state by electron impact is studied experimentally and theoretically. Electron-photon angular correlations measured at an incident electron energy of 200 eV and electron scattering angles between 5 • and 30 • are in reasonable agreement with theoretical predictions. The He + (2p) double-differential cross section shows structure due to interference effects arising from n 3 autoionization states decaying into this channel. The He + (2p) emission cross section from threshold to 300 eV is also reported and compared with previous data.
Results from an R-matrix with pseudo-states (RMPS) calculation are presented for electron-impact excitation of the (2s 2 2p) 2 P o → (2s2p 2 ) 4 P, (2s 2 2p) 2 P o → (2s 2 3s) 2 S and (2s 2 2p) 2 P o → (2s2p 2 ) 2 D transitions in neutral boron for incident energies from threshold up to 3 Ryd. At energies above the ionization threshold (0.61 Ryd), the inclusion of target continuum states in the close-coupling expansion is found to be crucial for accurate results to be obtained. Compared to previous first-order distorted-wave and R-matrix calculations with only discrete states, the present work represents a significant improvement in the prediction of the total excitation cross sections in the above energy range.
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