Experimental study of Velocity Changing Collisions on Coherent Population Trapping in Sodium

In this paper a detailed experimental study of Coherent Population Trapping (CPT) effect un sodium induced by a dye-laser operating in a three-mode re gime is presented and a detailed analysis of the role of velocity changing collisions is made. These collisions show a very small relaxation effect on the dark resonances which are visible even at high pressures. For the firs t time we demonstrate the persistence of the ground state coherence to pres sures up to one atmosphere for a relatively "heavy" buffer gas like argon. The experimental results have been compared with theory and a very good agr eement has: been obtained. Preliminary results on the effect of Na-N-2 coll isions on Coherent population Trapping are presented.