Molecular population trapping in Y- and Rhomb-type four-level systems

We report the first observation of molecular population trapping in a pair of coupled upper or coupled intermediate levels in Y-type or rhomb-type four-level systems, respectively. The demonstrations were done by means of equal-frequency two-photon excitations at about 587 and 640 run in sodium dimers. The mentioned coupled levels consist of a pair of wavefunction mixing levels due to spin-orbital perturbation with typical separation of a few GHz. The characteristic of the molecular trapping in a Y- type system was a strong extra UV excitation fluorescence peak against null visible emission in the middle between the two allowed two-photon absorption peaks. On the contrast, a pair of singlet-triplet mixing intermediate levels in a rhomb-type four-level system might provide significant destructive quantum interference for a two- photon transition, depending on their relative offsets. The characteristic of molecular population trapping in a rhomb-type four- level system was only two pieces of residual signals observable al the sides after completely eliminated of the center part of the two-photon lineshape.