Observation of positronium formation with inner orbital electrons for O$_{2}$, CO$_{2}$, C$_{2}$H$_{2}$, and N$_{2}$O using PsARS

We are investigating ortho- and para-positronium (Ps) formation for positrons interacting with simple gas molecules in a gas scattering cell using Ps annihilation ratio spectroscopy (PsARS)[1]. These measurements involve the detection of two gamma rays in coincidence for energy windows (a) centered at 511 keV resulting from the decay of short-lived (0.1 ns) para-Ps and the destruction of longer-lived (0.1 $\mu $s) ortho-Ps at the scattering cell walls, and (b) from 300 to 460 keV resulting from the three gamma decay of ortho-Ps. By taking the ratios of these signals versus positron impact energy we find we are able to compare the resulting curves with that obtained for argon as a reference. These comparisons reveal departures from the argon curve at well defined energies and are interpreted as Ps formation with inner orbital electrons. The fractions of the overall Ps formation cross-section due to inner orbital electrons are estimated to be as high as 30{\%} for O$_{2}$, 25{\%} for CO$_{2}$, 15{\%} for N$_{2}$O and 5{\%} for C$_{2}$H$_{2}$ near their respective threshold energies. [1] W.E. Kauppila, E.G. Miller, H.F.M. Mohamed, K. Pipinos, T.S. Stein, E. Surdutovich, Phys. Rev. Lett. 93, 113401 (2004).