Designing Molecular Eigenstates in a Four-level Lambda System

The interaction of single c.w. laser (coupling) field with a four-level lambda system containing two closely spaced upper levels (a and b) is studied as a means to attain quantum control. By adjusting the coupling field Rabi frequency and detuning, it is possible to create a target state of a prescribed admixture of a and b within the dressed state representation. We prove that the design of such a state requires a certain relation between the Rabi frequency and the detuning of the coupling laser, expressed by a specific ``control'' equation. We further show how the creation of the target eigenstate is revealed through the optical absorption spectrum. Electromagnetically induced transparency of an additional probe beam is also studied. The four-level lambda system gives rise to generally two dark resonances for a given coupling field detuning.