Coherent-population trapping in a highly degenerate open system

Coherent-population trapping, heretofore realized in closed systems, is also possible in highly degenerate open molecular systems. We consider a rovibrational molecular transition, where the ground magnetic m-sublevels are coupled to the corresponding upper states, and are therefore expected to be emptied after a few lifetimes. We show that upon excitation by elliptically polarized light, the population remains trapped in a coherent superposition of ground-state sublevels which does not interact further with the exciting light. Unlike the simple cases of linearly and circularly polarized light and of a closed three-level system (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.