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