Measurement of a Diamagnetic Shift in Atomic Hyperfine Structure

A diamagnetic shift in atomic hyperfine structure has been observed for the first time. The shift in the dipole hyperfine separation in ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$ has been measured by a high-field laser optical-pumping technique and found to be 1.65(16)×${10}^{-8\mathrm{}}$ Hz/${\mathrm{G}}^2$. In addition, the nuclear- to electronic—$g$-factor ratio, $\frac{g_I}{g_J}$, has been determined to be - 1.466490 93(11) × ${10}^{-4\mathrm{}}$, independent of magnetic field.