Gain in the concentrated solids occurs at {TAIL ~ energies} equal to or lower than the band gap photoluminescence, {HEAD ~ i.e., emission} 
Gain in the concentrated solids occurs at { TAIL ~ energies } equal to or lower than the band gap photoluminescence , { HEAD ~ i.e. , emission }
OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD