Frequency Shift of the Zero-Field Hyperfine Splitting ofCs133Produced by Various Buffer Gases

The pressure shift, caused by collisions with various noble gases in the zero-field ground-state hyperfine interaction of ${\mathrm{Cs}}^{133}$, has been measured by an optical transmission, microwave saturation method. A pressure shift to higher frequency was found in the case of the lighter gases, hydrogen, helium, nitrogen, and neon (+1900, +1600, +930, +650 cps/mm Hg) while a pressure shift to lower frequency was found in the case of the heavier gases, argon, krypton, and xenon (-250, -1300, -2400 cps/mm Hg).