  We propose and demonstrate pump-probe spectroscopy of rubidium absorption
which reveals the sub-Doppler hyperfine structure of the $^{5}$S$_{1/2}
\leftrightarrow$ $^{5}$P$_{3/2}$ (D2) transitions. The counter propagating pump
and probe lasers are independently tunable in frequency, with the probe
operating at the single-photon-level. The two-dimensional spectrum measured as
the laser frequencies are scanned shows fluorescence, Doppler-broadened
absorption dips and sub-Doppler features. The detuning between the pump and
probe lasers allows compensation of the Doppler shift for all atomic velocities
in the room temperature vapor, meaning we observe sub-Doppler features for all
atoms in the beam. We detail a theoretical model of the system which
incorporates fluorescence, saturation effects and optical pumping and compare
this with the measured spectrum, finding a mean absolute percentage error of
4.17\%. In the future this technique could assist in frequency stabilization of
lasers, and the single-photon-level probe could be replaced by a single photon
source.
