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 regime 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 first time we demonstrate the persistence of the ground state coherence to pressures up to one atmosphere for a relatively "heavy" buffer gas like argon. The experimental results have been compared with theory and a very good agreement has: been obtained. Preliminary results on the effect of Na-N-2 collisions on Coherent population Trapping are presented.