  We propose a method to generate a source of spin-polarized cold atoms which
are continuously extracted and guided from a magneto-optical trap using an
atom-diode effect. We show that it is possible to create a pipe-like potential
by overlapping two optical beams coupled with the two transitions of a
three-level system in a ladder configuration. With alkali-metal atoms, and in
particular with $^{87}$Rb, a proper choice of transitions enables both the
potential generation and optical pumping, thus polarizing the sample in a given
Zeeman state. We extend the Dalibard and Cohen-Tannoudji dressed-atom model of
radiative forces to the case of a three-level system. We derive expressions for
the average force and the different sources of momentum diffusion in the
resonant, non-perturbative regime. We show using numerical simulations that a
significant fraction of the atoms initially loaded can be guided over several
centimeters with output velocities of a few meters per second. This would
produce a collimated continuous source of slow spin-polarized atoms suitable
for atom interferometry.
