Isotope study of the nonlinear pressure shifts of $^{85}$Rb and $^{87}$Rb hyperfine resonances in Ar, Kr, and Xe buffer gases

Abstract

Measurements of the 0-0 hyperfine resonant frequencies of ground-state 85 Rb atoms show a nonlinear dependence on the pressure of the buffer gases Ar, Kr, and Xe. The nonlinearities are similar to those previously observed with 87 Rb and 133 Cs and presumed to come from alkali-metal-noble-gas van der Waals molecules. However, the shape of the nonlinearity observed for Xe conflicts with previous theory, and the nonlinearities for Ar and Kr disagree with the expected isotopic scaling of previous 87 Rb results. Improving the modeling alleviates most of these discrepancies by treating rotation quantum mechanically and considering additional spin interactions in the molecules. Including the dipolar-hyperfine interaction allows simultaneous fitting of the linear and nonlinear shifts of both 85 Rb and 87 Rb in either Ar, Kr, or Xe buffer gases with a minimal set of shared, isotope-independent parameters. To the limit of experimental accuracy, the shifts in He and N 2 were linear with pressure. The results are of practical interest to vaporcell atomic clocks and related devices. CONTENTS Data Availability Statement 17 S-I. Averaging over the direction of rotation 18 S-II. Estimates for the dipolar-and quadrupolar-hyperfine interactions 18 S-III. Pressure-gauge linearization corrections 18 S-IV. Positional shift in Camparo 18 S-V. Predictions for variation with transition and applied field 19 S-VI. Fit parameters for plotted curves 19