Frequency Shifts in Hyperfine Splitting of Alkalis Caused by Foreign Gases

The difference in the dispersion force between an alkali atom in a particular hyperfine level belonging to the groundstate and a perturbing molecule is computed. These asymptotic forces, if active alone, generally produce red shifts and suffice to account for the results obtained for the heavier buffer gases. Experimental data exhibit blue shifts for the lighter gases and therefore indicate that the net frequency shifts are the result of exchange as well as dispersion forces. The former cannot be determined theoretically. Therefore, a simple model is devised in which the difference in the forces is given a positive trend at distances of separation smaller than $d$, while beyond $d$ it is given the calculated form. The experimental data for all alkali-rare gas interactions can then be fitted by values of $d$ which, for the different foreign gases, are of the order of their gas-kinetic diameters.