Nonlinear collision shifts of the 0-0 hyperfine transition due to van der Waals molecule formation.

We consider the origin of nonlinear collision shifts for the 0-0 hyperfine transition in alkali/noble-gas systems due to van der Waals molecule formation. Developing a semi-empirical model, we describe the shift as arising from three fundamental interactions: (1) a fractional change in the alkali's valence electron density at the alkali nucleus, η, which affects the hyperfine contact term; (2) a mixing of p-wavefunction character into the alkali ground state (characterized by the probability for p-state character appearing in the perturbed wavefunction ξ ), which gives rise to an electric quadrupole term in the ground-state hyperfine splitting; and (3) an interaction of the alkali's valence electron with the magnetic field produced by molecular rotation, characterized by a magnetic field strength B. In addition to these molecular parameters, the model also depends on the formation rate of van der Waals molecules, kP, and the breakup rate of the molecules, kP, where P is the noble-gas pressure. Fitting the model to the Rb/Xe and Rb/Xe experimental data of McGuyer and co-workers (and taking previously measured values for k and B), we find that η = 9 × 10, ξ = 5 × 10, and k = 2.9×10 s/Torr.