Nonlinear pressure shifts of alkali-metal atoms in Xenon

We demonstrate that the microwave resonant frequencies of ground-state ⁸⁷Rb and ^133Cs atoms have a nonlinear dependence on the pressure of the buffer gas Xe. Surprisingly, though the frequency shifts of Rb and Cs in the heavy noble gases Ar, Kr, and Xe are all negative, the nonlinearity in Xe is of the opposite sign to the nonlinearities in Ar and Kr. This discrepancy suggests that the frequency shifts due to alkali-noble van der Waals molecules in Xe are opposite in sign to the those in Ar and Kr. The nonlinearity in Xe shows a reproducible deviation from a simple model used to describe the shifts in Ar and Kr, which suggests that the spin-rotation interaction plays a significant role in the frequency shifts due to molecules. No nonlinearities were observed with the gases He and N₂, as expected, and also Ne to within experimental error, which suggests that molecules do not form in Ne. Additionally, we present improved measurements of the shifts of Rb in Ar and Kr and of Rb and Cs in He and N₂.