  We observed electromagnetically induced transparency (EIT) and dark
fluorescence in a cascade three-level diatomic Lithium system using
Optical-Optical Double Resonance (OODR) spectroscopy. When a strong coupling
laser couples the intermediate state $A^{1}\Sigma^{+}_{u}(v=13, J=14)$ to the
upper 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 the
weak probe laser was tuned through the resonance transition between the ground
state $X^{1}\Sigma^{+}_{g}(v=4, J=15)$ and the excited state
$A^{1}\Sigma^{+}_{u}(v=13, J=14)$. The strong coupling laser makes an optically
thick medium transparent for the probe transition. In addition, The fact that
fluorescence from the upper state $G^{1}\Pi_{g}(v=11, J=14)$ was also dark when
both lasers were tuned at resonance implies that the molecules were trapped in
the ground state. We used density matrix methods to simulate the response of an
open molecular three-level system to the action of a strong coupling field and
a weak probe field. The analytical solutions were obtained under the
steady-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 excellent
agreement with the observed fluorescence spectra. We show that the coherence is
remarkably preserved even when the coupling field was detuned far from the
resonance.
