whereby {HEAD ~ laser} stimulated {TAIL ~ phtoelectron emissions} are differentiated over a threshold for detection 
stimulated
whereby { HEAD ~ laser } stimulated { TAIL ~ phtoelectron emissions } are differentiated over a threshold for detection
OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH