Fine-structure-changing collisions in atomic titanium

Abstract

We have measured cold titanium-helium collisions that cause transitions between the fine-structure levels of the $[3{d}^{2}4{s}^{2}]\phantom{\rule{0.3em}{0ex}}^{3}F_{J}$ electronic ground state of atomic titanium, over a temperature range from 5 to $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The $\mathrm{Ti}\text{\ensuremath{-}}\mathrm{He}$ inelastic collision cross section is significantly smaller than cross sections measured for collisions of non-transition-metal atoms with noble gas atoms. Our theoretical calculations of the inelastic cross sections reproduce the magnitude and temperature dependence of the measurements, and attribute the suppression of inelastic collisions to titanium's ``submerged'' $d$-shell valence electrons.