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We observed electromagnetically induced transparency (EIT) and darkfluorescence in a cascade three-level diatomic Lithium system usingOptical-Optical Double Resonance (OODR) spectroscopy. When a strong couplinglaser couples the intermediate state $A^{1}\Sigma^{+}_{u}(v=13, J=14)$ to theupper state $G^{1}\Pi_{g}(v=11, J=14)$ of $^7Li_2$, the fluorescence from both$A^{1}\Sigma^{+}_{u}$ and $G^{1}\Pi_{g}$ states was drastically reduced as theweak probe laser was tuned through the resonance transition between the groundstate $X^{1}\Sigma^{+}_{g}(v=4, J=15)$ and the excited state$A^{1}\Sigma^{+}_{u}(v=13, J=14)$. The strong coupling laser makes an opticallythick medium transparent for the probe transition. In addition, The fact thatfluorescence from the upper state $G^{1}\Pi_{g}(v=11, J=14)$ was also dark whenboth lasers were tuned at resonance implies that the molecules were trapped inthe ground state. We used density matrix methods to simulate the response of anopen molecular three-level system to the action of a strong coupling field anda weak probe field. The analytical solutions were obtained under thesteady-state condition. We have incorporated the magnetic sublevel (M)degeneracy of the rotational levels in the lineshape analysis and report $|M|$dependent lineshape splitting. The theoretical calculations are in excellentagreement with the observed fluorescence spectra. We show that the coherence isremarkably preserved even when the coupling field was detuned far from theresonance

Electromagnetically induced transparency and dark fluorescence in a cascade three-level diatomic lithium system