Observation of Quadrupole and Dipole Diamagnetic Shifts in Atomic Ground-State Hyperfine Structure

A high-precision laser optical-pumping experiment on the ${}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ ground state of atomic rubidium in magnetic fields of up to 8 T has led to the first observation of a magnetically induced quadrupole hyperfine interaction, in both ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$ and ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$. The quadrupole coupling constant was measured to be 7.0(6) × ${10}^{-4\mathrm{}}$ Hz ${\mathrm{T}}^{-2\mathrm{}}$ in ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$. The diamagnetic shift in dipole hyperfine structure was also measured in both ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$ and ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$. The fractional dipole shift was found to be independent of isotope with a value $\frac{\delta a}{a}=5.2\mathrm{}\left(2\right)\mathrm{}\times {10}^{-10\mathrm{}}$ ${\mathrm{T}}^{-2\mathrm{}}$, confirming and extending previous results for ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$ only.