COHERENT-POPULATION TRAPPING IN A HIGHLY DEGENERATE OPEN SYSTEM

Coherent-population trapping, heretofore realized in closed systems, i s also possible in highly degenerate open molecular systems. We consid er a rovibrational molecular transition, where the ground magnetic m-s ublevels are coupled to the corresponding upper states, and are theref ore expected to be emptied after a few lifetimes. We show that upon ex citation by elliptically polarized light, the population remains trapp ed in a coherent superposition of ground-state sublevels which does no t interact further with the exciting light. Unlike the simple cases of linearly and circularly polarized light and of a closed three-level s ystem (i.e. atoms), here the trapping level is not the one with \m\ = 0, J, but rather a combination of several states, which depends on the ellipticity of the exciting light.