Collisional excitation transfer between Rb(5P) states in 50-3000 Torr of He-4
Measurements of the mixing rates and cross sections for collisional excitation transfer between the 5P(1/2) and 5P(3/2) states of rubidium (Rb) in the presence of He-4 buffer gas are presented. Selected pulses from a high repetition rate, mode-locked femtosecond laser are used to excite either Rb state with the fluorescence due to collisional excitation transfer observed by time-correlated single-photon counting. The time dependence of this fluorescence is fitted to the solution of rate equations which include the mixing rate, atomic lifetimes and any quenching processes. The variation in the mixing rate over a large range of buffer gas densities allows the determination of both the binary collisional transfer cross section and a three-body collisional transfer rate. We do not observe any collisional quenching effects at He-4 pressures up to 6 atm and discuss in detail other systematic effects considered in the experiment.