Frequency shifts and line broadenings in collisions between Rydberg and ground-state alkali-metal atoms

The technique of Doppler-free two-photon spectroscopy was used to measure rates for line broadenings and frequency shifts of Rydberg levels with pressure. Measurements were made of spectra of ${}_{}{}^2{}_{}{}^{}\mathrm{S}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{D}$ states (with n∼10–40) for ${\mathrm{Rb}}^{\mathrm{*}\mathrm{*}}$+Rb and ${\mathrm{K}}^{\mathrm{*}\mathrm{*}}$+K, and of ${}_{}{}^2{}_{}{}^{}\mathrm{S}$ states (with n=14–29) for ${\mathrm{K}}^{\mathrm{*}\mathrm{*}}$+Rb. Large oscillations are observed in both broadening and shift rates for n∼20–30 in all series except for the ${}_{}{}^2{}_{}{}^{}\mathrm{D}$ series in ${\mathrm{Rb}}^{\mathrm{*}\mathrm{*}}$+Rb. Overall rates for broadening and shift are compared with a theory based on the ‘‘free-electron’’ model. The observed oscillations are inconsistent with theories based on the resonance scattering of the Rydberg electron by the perturbing atom. A possible alternative mechanism related to interference in l-mixing collisions is suggested.