Spectroscopy of the ⁸⁵Rb 4D_3/2 state for hyperfine-structure determination

We report a measurement of the hyperfine-structure constants of the Rb-85 4D(3/2) state using two-photon optical spectroscopy of the 5S(1/2) -> 4D(3/2) transition. The spectra are acquired by measuring the transmission of the low-power 795 nm lower-stage laser beam through a cold-atom sample as a function of laser frequency, with the frequency of the upper-stage, 1476 nm laser fixed. All 4 hyperfine components of the 4D(3/2) state are well-resolved in the experimental data. The dominant systematic is the light shift from the 1476 nm laser, which is addressed by extrapolating line positions measured for a set of 1476 nm laser powers to zero laser power. The analysis of our experimental data yields both the magnetic-dipole and electric-quadrupole constants for the (85)Rb4D(3/2) level, without using earlier hyperfine measurements of other atomic levels. The respective results, A =7.419(35) MHz and B =4.19(19) MHz, are discussed in context with previous works. Our investigation may be useful for optical atomic clocks for precision metrology and emerging atom-based quantum technologies, all-infrared excitation of Rb Rydberg levels, and molecular physics.