  Our work is based on the collision-induced coherence of two decay channels
along two optical transitions.The quantum interference of pumping processes
creates the dark state and the more atoms are pumped in this collision-induced
dark state the stronger the suppression of the spontaneous emission. The
efficiency of this suppression is quantified by putting it in comparison with
the spontaneous emission on the ultraviolet transition which proceeds in a
regular fashion. The branching ratio of these two(visible and ultraviolet)
transitions is introduced as the effective measure of the degree of the
suppression of the spontaneous emission on the visible transition. Our
preliminary calculations show that a significant decrease of the branching
ratio with increase of electron densities is reproduced in the theory.
