Electron-impact ionization cross section of rubidium

Abstract

A theoretical model for electron-impact ionization cross section has been applied to Rb and the theoretical cross section (from the threshold to 1 keV in incident energy) is in good agreement with the recent experimental data obtained using Rb atoms trapped in a magneto-optical trap. The theoretical model, called the binary-encounter--dipole (BED) model, combines a modified Mott cross section with the high-energy behavior of Born cross sections. To obtain the continuum dipole oscillator strength $df/dE$ of the $5s$ electron required in the BED model, we used Dirac-Fock continuum wave functions with a core polarization potential that reproduced the known position of the Cooper minimum in the photoionization cross section. For inner-shell ionization, we used a simpler version of $df/dE$, which retained the hydrogenic shape. The contributions of the $4\stackrel{\ensuremath{\rightarrow}}{p}4d$, $5s$, and $5p$ autoionizing excitations were estimated using the plane-wave Born approximation. As a by-product, we also present the dipole oscillator strengths for the $5\stackrel{\ensuremath{\rightarrow}}{s}{\mathrm{np}}_{1/2}$ and $5\stackrel{\ensuremath{\rightarrow}}{s}{\mathrm{np}}_{3/2}$ transitions for high principal quantum numbers $n$ near the ionization threshold obtained from the Dirac-Fock wave functions with the same core polarization potential as that used for the continuum wave functions.