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 or partial two-photon Doppler broadening and crossover 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) effects followed by the subtraction of the broad background of the two-photon spectrum. Since the VIPO and VSS effects 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 $6{P}_{3/2}$ state in Rb are well resolved. The double-resonance experiment is conducted on the $5{S}_{1/2}\ensuremath{\rightarrow}5{P}_{3/2}$ strong transition (at 780 nm) and $5{S}_{1/2}\ensuremath{\rightarrow}6{P}_{3/2}$ weak transition (at 420 nm) at room temperature.