Positronium–atom collisions

Abstract

New results are presented for Ps(1s) scattering by H(1s), He(1 1 S) and Li(2s). Calculations have been performed in a coupled state framework, usually employing pseudostates, and allowing for excitation of both the Ps and the atom. In the Ps(1s)-H(1s) calculations the H − formation channel has also been included using a highly accurate H − wave function. Resonances resulting from unstable states in which the positron orbits H − have been calculated and analysed. The new Ps(1s)-He(1 1 S) calculations still fail to resolve existing discrepancies between theory and experiment at very low energies. The possible importance of the Ps − formation channel in all three collision systems is discussed.

Thermalization of positronium in helium

“…[3] Improvements in this theory have still not resolved the disagreements. [10] Above 10 eV there are Ps beam measurements of the total cross section which are in reasonable accord with all three theories but which, at 10 eV, show a tantalizing turn-down towards the Michigan measurements. [11] Unfortunately the beam experiments find difficulty penetrating into the region below 10 eV.…”

Positron and positronium scattering

“…www.pss-c.com Figure 6(b) shows the momentum transfer cross section for He both in the frozen target approximation (Fig. 4) and in an approximation allowing for target excitation [29]. Allowing for target excitation does move the cross section towards the experiments of Canter et al, Rytsölä et al and Skalsey et al but there is still substantial disagreement.…”

“…(a) Ps(1s)+H(1s) total cross section [28]. (b) Ps(1s)+He(1 1 S) momentum transfer cross section [29]; experiment: square, Nagashima et al [20]; up triangle, Canter et al [18]; down triangle, Rytsölä et al [30]; circle, Skalsey et al [31]. Figure 5(a) compares frozen target calculations [16,17] with beam measurements of the total cross section for He [23][24][25] at impact energies of 10 eV and above.…”

“…Electronic spin singlet P-wave cross section for Ps(1s)-H(1s) elastic scattering [29]. Curves: dash-dot, frozen target approximation; dashed, approximation allowing for target excitation; solid, approximation allowing for target excitation and also including the H − channel.…”

“…In Fig. 8(a) we show the latest results [29] for the total cross section. This calculation allows for target excitation and includes the H − channel.…”