Excited-State Positronium Formation from Helium, Argon, and Xenon

The cross sections for the formation of positronium in the $2P$ state in collisions of positrons with He, Ar, and Xe atoms have been determined by measuring coincidences between the remnant ion and the Lyman-$\alpha$ photon from positronium. The maximum fractional contributions of these to the total Ps formation cross sections increase from approximately $0.06\pm 0.01$ in He to $0.12\pm 0.04$ in Ar and $0.26\pm 0.09$ in Xe. In the case of He, good agreement is found with a coupled-state calculation; for Ar and Xe, measurements are compared with a distorted-wave Born approximation.

Figure 1

The present $Q_{\mathrm{Ps}}(2P)$ for He (circles) compared with available theories: solid curve, Ref. 19; dotted line, Ref. 20; chain curve, Ref. 16; dashed curve, Ref. 17; double chain curve, Ref. 18. The error bars include the statistical errors on the yields and the uncertainties on the normalization factors. Above the main figure is the percentage contribution of $Q_{\mathrm{Ps}}(2P)$ to the $Q_{\mathrm{Ps}}$(all $n$) of Ref. 29. In the inset, $Q_{\mathrm{Ps}}$(all $n$) are displayed: circles, Ref. 29; lines, as in main figure.

Figure 2

The present $Q_{\mathrm{Ps}}(2P)$ for Ar (circles) compared with theory [21] (line). Error bars as per Fig. 1. Above the main figure is the percentage contribution of $Q_{\mathrm{Ps}}(2P)$ to the $Q_{\mathrm{Ps}}$ (all $n$) of Ref. 22 compared with the corresponding calculation [21, 24]. In the inset, $Q_{\mathrm{Ps}}$ (all $n$) are displayed: circles, Ref. 22; lines, Refs. 21, 24.

Figure 3

As per Fig. 2 but for Xe.

Figure 4

Present $Q_{\mathrm{Ps}}(2P)$ for Ar and Xe (circles), the limits of $Q_{\mathrm{Ps}}(n>1)$: gray region, Ref. 22; line, DWBA results [24].