Optical spectroscopy of the 5p3/2 → 6p1/2 electric dipole-forbidden transition in atomic rubidium

We present the first evidence of excitation of the 5p(3/2) -> 6p(1/2) electric dipole-forbidden transition in atomic rubidium. The experiments were done in a rubidium vapor cell using Doppler-free optical-optical double resonance spectroscopy with counter-propagating beams, with a similar scheme to the excitation of the other fine transition 6p(3/2) in the 6p(J) manifold reported in Ponciano-Ojeda et al (2015 Phys. Rev. A 92 042511). The 5p(3/2) -> 6p(1/2 )dipole-forbidden excitation occurs at a wavelength lambda approximate to 917.5 nm. The spectra showed the two main hyperfine states of the 6p(1/2) manifold, and also resolved up to three satellites that appear due to effects of selection of velocities in thermal rubidium atoms. The polarization dependence of the relative intensities of the hyperfine lines of the decay fluorescence is also investigated. The data show that the individual line intensities and also the total cross-section have a strong dependence on the polarization direction. Experimental data for different polarization configurations of the light beams used in this two-photon spectroscopy 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, which confirms the results found for the 5p(3/2) -> 6p(3/2) forbidden transition. The results presented here complete the information about the 5p(3/2) -> 6p(J), electric dipole-forbidden transition in rubidium.