We present an approximate theory of intramultiplet mixing, relaxation, and polarization transfer in excited alkali atoms, valid for all magnitudes of the fine-structure splitting. The theory is developed as the generalization of the previously postulated theory of fine-structure mixing induced in adiabatic and quasiresonant collisions. In the adiabatic region, the theory predicts a change in the sign of the coefficient for polarization transfer between the components j = 1/2 and,j = 3/2 of the 2P resonance doublet, as the Massey parameter for this transition changes.
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.