POSITRON SCATTERING BY POTASSIUM

Coupled-state calculations in a K(4s,4p,5s,5p,3d) + Ps(1s,2s,2p,3s,3p, 3d) approximation are reported for positron scattering by ground-state potassium in the energy range 0.5-60.0 eV. Comparison is made with th e earlier work of Hewitt et al (1993, 1994) in a K(4s,4p, 5s, 5p) + Ps (ls, 2s, 2p) approximation. For the first time cross sections for posi tronium formation in n = 3 states are obtained. We support the conclus ion of Hewitt et al that above about 4 eV positronium formation is mai nly into excited states, although we disagree significantly on the dis tribution of the excited state population. Fairly good agreement is ob tained with the measurements of Zhou et al (1994) for total positroniu m formation when n(3) scaling is used to estimate the amount of positr onium in states with n greater than or equal to 4. Our cross sections for elastic scattering, 4s-4p, 4s-5s and 4s-5p excitation of potassium are in relatively good agreement with those of Hewitt er al but our c alculations also emphasize the importance of the 4s-3d excitation whic h is omitted in their approximation. Atom excitation to the n = 5 leve l is found to be a relatively minor process, Good agreement is obtaine d with the total cross section measurements of Kwan er al (1991) and P arikh er al (1993) when these are normalized upwards by 10% and correc tions made for near forward elastic scattering. This upward normalizat ion lies within the quoted overall 21% error on the measurements. Belo w 4 eV the calculated total cross section is dominated by elastic scat tering while at high energies 4s-4p excitation is the main contributor . In the vicinity of 6 eV, and consistent with the measurements of Par ikh et al (1993), the total cross section displays a pronounced peak w hich derives largely, although not totally, from positronium formation . Positronium formation is not important above about 30 eV.