  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.

Atomic Physics