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