  The two-photon transition $5S-5P-5D$ in rubidium vapor is investigated by
detecting the fluorescence from the $6P_{3/2}$ state when the atomic system is
excited by the combined action of a cw diode laser and a train of ultrashort
pulses. The cw-laser plays a role as a velocity-selective filter and allows for
a spectroscopy over a large spectral range including the $5D_{3/2}$ and
$5D_{5/2}$ states. For a counterpropagating beam configuration, the response of
each atomic velocity group is well characterized within the Doppler profile,
and the excited hyperfine levels are clearly resolved. The contribution of the
optical pumping to the direct two-photon process is also revealed. The results
are well described in a frequency domain picture by considering the interaction
of each velocity group with the cw laser and the modes of the frequency comb.
