For years, inelastic, fine-structure-changing collisions between excited alkali-metal atoms and noble-gas perturbers have been a source of information on the alkali-metal--noble-gas interaction. Here, we consider the role of elastic fine-structure-mixing collisions on alkali-metal absorption cross sections. Specifically, we consider the four optical hyperfine transitions of the
D1 absorption spectrum for
87Rb, and in particular the doublet corresponding to $5^{2}S_{1/2}({F}_{g}=2)\ensuremath{-}5^{2}P_{1/2}({F}_{e}=1,2)$ transitions. Although ideally the amplitudes of the doublet's absorption cross sections are identical, we find that fine-structure mixing of
D2 oscillator strength into the
D1 spectrum breaks the cross sections' equality and leads to doublet asymmetry. We obtain a fine-structure mixing parameter $\ensuremath{\lambda}$ of $(5.4\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}$, and estimate a
52P1/2 hyperfine-splitting density shift $\ensuremath{\zeta}$ in the
87Rb-Xe system of $+0.71\ifmmode\pm\else\textpm\fi{}0.03\phantom{\rule{0.16em}{0ex}}\mathrm{GHz}/\mathrm{amagat}$. These results have implications for spectroscopic applications of the
87Rb isoclinic point, which we consider as part of our conclusions.
The cross sections for fine-structure transition (FST) of Rb(7 2 D ) atoms induced by collision with molecules of H 2 and CO have been experimentally determined by the method of selective stepwise excitation of Rb atoms and observation of the resulting fluorescence. The cross sections (in units of 10 -14 cm 2 ) for Rb(7 2 D )-H 2 are σ fs *(7 2 D 5/2 → 7 2 D 3/2 ) = 0.16 ± 0.03, σ fs *'(7 2 D 3/2 → 7 2 D 5/2 ) = 0.24 ± 0.05, for Rb(7 2 D )-CO: σ fs * = 0.10 ± 0.06 and σ fs * ' = 0.15 ± 0.09. The cross sections σ tr * for transfer out of the 7 2 D doublet were also determined.