Argon-induced hyperfine frequency shift in potassium

The hyperfine spectrum of the Van der Waals molecule KAr is analyzed assuming an isotropic perturbation of the potassium hyperfine constant. The analysis yields the hyperfine frequency shift in KAr as a function of the internuclear separation between 9 and 18 bohr. The shift is positive at short range, $R<\mathrm{}12\mathrm{}$ bohr, and negative at longer range. The results are compared to theoretical estimates of the shift, and are used to calculate the pressure shift of optically pumped potassium in argon.