Resonant scattering of positronium as a quasifree electron

In order to clarify the physics underlying the observations of the electronlike behavior of positronium (Ps) and its resonant scattering from ${\mathrm{CO}}_2$, we have measured the $\text{Ps}+{\mathrm{N}}_2$ total cross section and found it also to exhibit significant structure. Analysis of the resonances reveals that Ps is distorted in the collisions and classical trajectory Monte Carlo calculations indicate that the electron is on average closer to the target than the positron, which may in turn bind resonantly to the ensuing temporary negative ion. This description of the nature of Ps resonances agrees with long-standing theoretical predictions.

Figure 1

Total cross sections for equivelocity Ps, positrons, and electrons colliding with ${\mathrm{N}}_2$. Here the $•$ represent the $Q_{\mathrm{T}}^{\mathrm{Ps}}$ measurements from the present work and the $\circ$ those from [16], while —— shows the corresponding $Q_{\mathrm{T}}^{+}$ from [36] and — — — the $Q_{\mathrm{T}}^{-}$ from [33].

Figure 2

Total cross sections for ${\mathrm{N}}_2$ and ${\mathrm{CO}}_2$ versus energy for Ps and electron collisions. For ${\mathrm{N}}_2$, the $•$ represent the $Q_{\mathrm{T}}^{\mathrm{Ps}}$ measurements from the present work and the $\circ$ those from [16], while — — — shows $Q_{\mathrm{T}}^{-}$ from [33]. For ${\mathrm{CO}}_2$, the $•$ represent the $Q_{\mathrm{T}}^{\mathrm{Ps}}$ measurements from [15] and — — — shows $Q_{\mathrm{T}}^{-}$ from [42].

Figure 3

Comparison of $Q_{\mathrm{T}}^{\mathrm{Ps}}$ with the convoluted $Q_{\mathrm{T}}^{-}$ for collisions with ${\mathrm{CO}}_2$ (left) and ${\mathrm{N}}_2$ (right). In the top panels open circles show $Q_{\mathrm{T}}^{\mathrm{Ps}}$ measurements for ${\mathrm{CO}}_2$ from [15] and for ${\mathrm{N}}_2$ from [16] and the present work; dashed lines show $Q_{\mathrm{T}}^{-}$ measurements for ${\mathrm{CO}}_2$ from [42] and for ${\mathrm{N}}_2$ from [33]; the solid line shows convoluted $Q_{\mathrm{T}}^{-}$ . Shown in the bottom panels are the convoluted (lines) and measured (circles) resonance peaks for Ps impact after subtracting the nonresonant contribution. For each target, the Ps data have been shifted to lower energies by the appropriate $\mathrm{\Delta }{E}_{\mathrm{res}}$.

Figure 4

Comparison of the convoluted $Q_{\mathrm{T}}^{-}$ with $Q_{\mathrm{T}}^{\mathrm{Ps}}$ for collision with ${\mathrm{N}}_2$ with respect to the peak at $\sim 5\mathrm{eV}$ (left) and $\sim 8\mathrm{eV}$ (right). The legend is the same as for Fig. 3.

Figure 5

Time evolution of the longitudinal velocity distribution of the electron (dashed curve) and the positron (solid curve) predicted by CTMC for Ps on oxygen collisions at the resonance velocity $v_0=0.528$ a.u. (see the text).

Figure 6

Contour plot of the distribution of the $(y,z)$ coordinates of the (a) electrons and (b) positrons in collisions of Ps with oxygen atoms at time $t=-1$ a.u.