Nonlinear Pressure Shifts of Alkali-Metal Atoms in Inert Gases

Precise measurements show that the microwave resonance frequencies of ground-state Rb or Cs atoms have a nonlinear dependence on the pressure of the buffer gases Ar and Kr. No nonlinearities were observed in the gases He or N₂. These observations strongly suggest that the nonlinearities are due to the van der Waals molecules that form in Ar and Kr, but not in He or N₂. The nonlinear part of the shifts is largest in the pressure range of a few tens of torr, similar to the operating pressures of gas-cell atomic clocks. The observed shifts are very well described by a simple function, parametrized by the effective three-body formation rate of molecules and by the effective product of the collisionally limited lifetime times the shift of the hyperfine coupling coefficient in the molecule.