Dependence of CRIR on Galactocentric radius, R_g, for diffuse atomic material in the Galactic disk. Magenta diamonds: values of ﹩{\mathrm{log}}_{10}[{\zeta }_{p}({\rm{H}})/{n}_{50}]﹩ determined from observations of OH⁺, H₂O⁺, and ArH⁺. Red squares: values of ﹩{\mathrm{log}}_{10}[{\zeta }_{p}({\rm{H}})/{n}_{50}]﹩ determined from measurements of OH⁺ and H₂O⁺ alone. All values are computed for an assumed ﹩{\chi }_{\mathrm{UV}}/{n}_{50}﹩ of 1.