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