Resonance flourescence in atomic coherent systems: spectral features

We study resonance flourescence in a four level ladder system and illustrate some novel features due to quantum interference and atomic coherence effects. We find that under three photon resonant conditions, in some region of the parameter space of the rabi frequencies $\Omega_1,\Omega_2,\Omega_3$, emission is dominantly by the level 4 at the line center even though there is an almost equal distribution of populations in all the levels. As one increases $\Omega_3$ with $\Omega_1 and \Omega_2$ held fixed, the four level system 'dynamically collapses' to a two level system. The steady state populations and the the resonance flourescence from all the levels provide adequate evidence to this effect.