Completing the optical spectroscopy of the $6p_{J}$ manifold: the $5p_{3/2}\rightarrow 6p_{1/2}$ electric dipole forbidden transition in atomic rubidium

We present the first evidence of excitation of the $5p_{3/2} \rightarrow 6p_{1/2}$ electric dipole-forbidden transition in atomic rubidium. The experiments were carried out in a rubidium vapor cell using Doppler-free optical-optical double-resonance spectroscopy with counter-propagating beams. This a similar scheme to the excitation of the other fine transition $J=3/2$ in the $6p_{J}$ manifold previously reported. A $5s_{1/2} \rightarrow 5p_{3/2}$ electric dipole preparation step using a diode laser locked to the maximum F cyclic transition of the $D_2$ line is used to prepare the atoms in the first excited state. This is followed by the $5p_{3/2} \rightarrow 6p_{1/2}$ dipole-forbidden excitation, in a two-photon ladder ($\Xi$) excitation scheme. Production of atoms in the $6p_{1/2}$ excited state is verified by detection of the $421$ nm fluorescence that results from direct decay into the $5s_{1/2}$ ground state. The polarization dependence of the relative intensities of the lines of the decay fluorescence is also investigated. Experimental data for different polarization configurations of the light beams are compared with the results of calculations that consider a strong atom-field coupling in the preparation step, followed by a weak electric quadrupole excitation and the blue fluorescence decay emission. Good agreement between experiment and this three-step model is found in the case of linear-linear polarizations in both isotopes.