  The microwave resonant frequencies of ground-state 87Rb and 133Cs 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 der Waals 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 in addition to the hyperfine-shift interaction in the molecules. To
the limit of our experimental accuracy, the shifts of 87Rb and 133Cs in He, Ne,
and N2 were linear with pressure.
