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 ²S and ²D states (with n{~}10--40) for Rb**+Rb and K**+K, and of ²S states (with n=14--29) for K**+Rb. Large oscillations are observed in both broadening and shift rates for n{~}20--30 in all series except for the ²D series in Rb**+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.