Hyperfine frequencies of 87Rb and 133Cs atoms in Xe gas

The microwave resonant frequencies of ground-state Rb-87 and Cs-133 atoms in Xe buffer gas are shown to have a relatively large nonlinear dependence on the Xe pressure, presumably because of RbXe or CsXe van derWaals molecules. The nonlinear shifts for Xe are opposite in sign to the previously measured shifts for Ar and Kr, even though all three gases have negative linear shifts. The Xe data show striking discrepancies with the previous theory for nonlinear shifts. Most of this discrepancy is eliminated by accounting for the spin-rotation interaction, gamma N . S, in addition to the hyperfine-shift interaction, delta A I . S, in the molecules. To the limit of our experimental accuracy, the shifts of Rb-87 and Cs-133 in He, Ne, and N-2 were linear with pressure.