{TAIL ~ Gas molecules} in a gas stream containing one of the isotopes are selectively excited to a different electronic state while leaving the other gas molecules in {HEAD ~ their original ground state} 
{ TAIL ~ Gas molecules } in a gas stream containing one of the isotopes are selectively excited to a different electronic state while leaving the other gas molecules in { HEAD ~ their original ground state }
TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD