  We consider the coherent population trapping phenomenon in a thermal sodium
atomic beam. We compare the different coherent population trapping schemes that
can be established on the D1 line using the Zeeman sublevels of a given ground
hyperfine state. The coherent population trapping preparation is examined by
means of a Hanle effect configuration. The efficiency of the coherent
population trapping phenomenon has been examined in presence of optical pumping
into hyperfine levels external to those of the excited transition. We show that
both the contrast and the width of the coherent population trapping resonance
strongly decrease when the optical pumping rate is increased. In the
experiment, the loss rate due to optical pumping has been controlled by means
of a laser repump of variable intensity.
