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Resolving closely spaced levels for Doppler mismatched double resonance

In this paper, we present experimental techniques to resolve the closely spaced hyperfine levels of a weak transition by eliminating the residual/partial two-photon Doppler broadening and cross-over resonances in a wavelength mismatched double resonance spectroscopy. The elimination of the partial Doppler broadening is based on velocity induced population oscillation (VIPO) and velocity selective saturation (VSS) effect followed by the subtraction of the broad background of the two-photon spectrum. Since the VIPO and VSS effect are the phenomena for near zero velocity group atoms, the subtraction gives rise to Doppler-free peaks and the closely spaced hyperfine levels of the $6\text{P}_{3/2}$ state in Rb are well resolved. The double resonance experiment is conducted on $5\text{S}_{1/2}\rightarrow5\text{P}_{3/2}$ strong transition (at 780~nm) and $5\text{S}_{1/2}\rightarrow6\text{P}_{3/2}$ weak transition (at 420~nm) at room temperature.