Cross sections for ionization of metastable rare-gas atoms (Ne*, Ar*, Kr*, Xe*) and of metastableN2*, CO* molecules by electron impact

Abstract

Cross sections for the collisional ionization of the metastable atoms Ne*, Ar*, Kr*, and Xe* by electrons with impact energy $E$ in the range $6<E<\mathrm{}250\mathrm{}$ eV are determined in the Born and binary-encounter approximations. For low energies $\epsilon$ of ejection, the $s-d$ and $s-f$ bound-free transitions dominate the associated form factor, while transitions to continuum states with progressively higher angular momentum gain importance with increasing $\epsilon$. While up to nine partial waves are normally sufficient for convergence of the bound-free form factor at a given energy $\epsilon$ and momentum change $K$ of the ejected electron, as many as 30 are required for those $K$ in the vicinity of the Bethe ridge at large $\epsilon$. These properties are the origin of the overall closeness obtained between the Born and binary-encounter cross sections. Also inner-shell ionization, as described by the binary-encounter treatment, becomes increasingly important as the target atom becomes more complex. Cross sections for ionization of metastable ${\mathrm{N}}_2$* and CO* are also determined. Good agreement with available measurements (for Ne* and Ar*) is obtained.