Use of the 5P3/2→6P3/2 electric dipole forbidden transition in Rb as a non-perturbing probe of atom dynamics in an operating magneto-optical trap

Results of spectroscopy experiments using the 5P(3/2)-+ 6P(3/2) electric dipole forbidden transition in cold rubidium atoms are presented. Production of this forbidden transition is detected by observing the emission of the 420 nm fluorescence photons that result from the decay of the 6P(3/2) state into the ground state. The experiments are performed under the steady state operation conditions of a magneto-optical trap (MOT), and thus provide non-perturbing information on the atom-light system. The hyperfine structure of the 6P(3/2) level is completely resolved, and the fluorescence peaks show the expected Autler-Townes (AT) splitting of the emission lines. This hyperfine structure is used to calibrate the frequency scale of all spectra recorded. A combination of this absolute frequency scale and an expression for the AT profile allows an absolute determination of the effective Rabi frequency and detuning of the MOT. The behavior of these AT doublets is studied as a function of frequency detuning and also as a function of the trapping light intensity. The experiments also take full advantage of the strong polarization dependence of the relative intensities of the hyperfine fluorescence components. This study results in a sensitive probe of the relative populations of the magnetic sublevel projections relative to the trapping magnetic field gradient. An almost isotropic population distribution was found, but small deviations from isotropy could be determined with this electric dipole forbidden probe.