The electron hole pairs trapped within the active layer sensitize {HEAD ~ the activator ions}, causing {TAIL ~ the activator ions} to emit light 
causing
The electron hole pairs trapped within the active layer sensitize { HEAD ~ the activator ions } , causing { TAIL ~ the activator ions } to emit light
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH