Positron-impact ionization, positronium formation, and electronic excitation cross sections for diatomic molecules

Abstract

Absolute measurements are presented for the positron-impact cross sections for positronium formation, direct ionization, and total ionization of the diatomic molecules ${\mathrm{N}}_{2}$, CO, and ${\mathrm{O}}_{2}$, in the range of energies from threshold to $90\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Cross sections for the electronic excitation of the $a\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}$ and ${a}^{\ensuremath{'}}\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}$ state in ${\mathrm{N}}_{2}$ and the $A\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}$ state in CO near threshold are also presented. The experiment uses a cold, trap-based positron beam and the technique of studying positron scattering in a strong magnetic field. In ${\mathrm{O}}_{2}$, a feature previously seen in the total ionization cross section is observed in both the positronium formation and total ionization cross sections. The possible origin of this feature and its relationship to positron-induced dissociation is discussed. In ${\mathrm{N}}_{2}$, the near-threshold electronic excitation cross section is larger than that for positronium formation. This likely explains the relatively high efficiency of this molecule when used for buffer-gas positron trapping.