Observation of the 5 p3 /2→6 p3 /2 electric-dipole-forbidden transition in atomic rubidium using optical-optical double-resonance spectroscopy

Direct evidence of excitation of the 5 p_{3 /2}→6 p_{3 /2} electric-dipole-forbidden transition in atomic rubidium is presented. The experiments were performed in a room-temperature rubidium cell with continuous-wave external cavity diode lasers. Optical-optical double-resonance spectroscopy with counterpropagating beams allows the detection of the nondipole transition free of Doppler broadening. The 5 p_{3 /2} state is prepared by excitation with a laser locked to the maximum F cyclic transition of the D₂ line, and the forbidden transition is produced by excitation with a 911 nm laser. Production of the forbidden transition is monitored by detection of the 420 nm fluorescence that results from decay of the 6 p_{3 /2} state. Spectra with three narrow lines (≈13 MHz FWHM) with the characteristic F -1 , F , and F +1 splitting of the 6 p_{3 /2} hyperfine structure in both rubidium isotopes were obtained. The results are in very good agreement with a direct calculation that takes into account the 5 s →5 p_{3 /2} preparation dynamics, the 5 p_{3 /2}→6 p_{3 /2} nondipole excitation geometry, and the 6 p_{3 /2}→5 s_{1 /2} decay. The comparison also shows that the electric-dipole-forbidden transition is a very sensitive probe of the preparation dynamics.