Diamagnetic Shifts in Atomic Hyperfine Structure.

A series of precision measurements of the hyperfine structure of the 5('2)S(, 1/2) ground state of two isotopes of atomic rubidium in magnetic fields of up to 8 T was made using a laser optical pumping technique. Observed departures from the predictions of the Breit-Rabi formula include the first measurement of a magnetically induced quadrupole hyperfine shift. A field dependence of the effective value of the nuclear-to-electronic g-factor ratio g(,I)/g(,J) was revealed, and the previous observation of a diamagnetic shift in dipole hyperfine structure in ('85)Rb was confirmed and extended to ('87)Rb. In order to distinguish among these effects, all of the (DELTA)m(,I) = (+OR-)1 nuclear Zeeman transitions in both ('85)Rb and ('87)Rb were measured at each field. Four evacuated, wax -coated sample cells were used, containing natural isotopic abundance rubidium vapor. Perturbation theory is used to examine the effects of an external magnetic field on the ground state hyperfine structure of an alkali atom. A Hamiltonian is developed and evaluated to yield a modified Breit-Rabi formula which provides an excellent fit to the experimental data. Approximate calculations performed using this model are in good agreement with the dipole and quadrupole results. The shift in the g-factor ratio, on the other hand, is much larger than predicted.