CROSS-CHANNEL COUPLING IN POSITRON-ATOM SCATTERING

Coupled-state calculations including positronium channels are reported for positron scattering by atomic hydrogen, lithium and sodium. Integ rated cross sections and total cross sections are presented for all th ree atoms. For lithium differential cross sections are also given. Thr oughout, comparison is made between results calculated with and withou t inclusion of the positronium channels. S-wave cross sections for pos itron scattering by atomic hydrogen in the Ps(1s, 2s, 2p) + H(1s, 2s, 2p) approximation show the high energy resonance first observed by Hig gins and Burke in the coupled-static approximation. This resonance has now moved up to 51.05 eV and narrowed in width to 2.92 eV. Other pron ounced structure is seen in the S-wave cross sections between 10 and 2 0 eV; it is tentatively suggested that this structure may be due to th e formation of a temporary pseudo-molecular collision complex. Results calculated in the Ps(1s, 2s, ($) over bar 3s, ($) over bar 4s, 2p, ($ ) over bar 3p, ($) over bar 4p, ($) over bar 3d, ($) over bar 4d) + H( 1s, 2s, ($) over bar 3s, ($) over bar 4s, 2p, ($) over bar 3p, ($) ove r bar 4p, ($) over bar 3d, ($) over bar 4d) approximation show converg ence towards accurate values in the energy region below and in the Ore gap. Contrary to previous work on lithium using only an atomic basis, it is found that coupling to the 3d state of lithium is not so import ant when positronium channels are included; this is because a mixed ba sis of atom and positronium states gives a more rapidly convergent app roximation than an expansion based on atom states alone. The threshold behaviour of the elastic cross section and the Ps(1s) formation cross section for lithium is investigated. Results in the Ps(1s, 2s, 2p) Na(3s, 3p) approximation for sodium show good agreement with the total cross section measurements of Kwan et al.