Enhancement of Rb fine-structure transfer in He and Ar due to three-body collisions

Using ultrafast laser excitation and time-correlated single- photon counting techniques we have measured the collisional mixing rates between the rubidium $5^{2}P$ fine-structure levels in the presence of He and Ar inert gases. The mixing rates and collisional cross sections are determined from the time-dependence of the fluorescence observed from collisional transfer. A nonlinear dependence of the mixing rate with $^{4} $He density is observed. This nonlinearity corresponds to Rb fine-structure transfer primarily due to binary collisions with $^{4}$He at densities of $\leq$ 10$^{19}$ cm$^{-3}$, while at greater densities three-body collisions become significant. These interactions can be between bound, quasibound, and free atoms which we discuss. Recent measurements of Rb-Ar fine- structure transfer will also be presented.