Abstract: The isotopes of a gas are separated by using a circularly polarized laser to excite a lower energy level to higher split intermediate energy levels, a second laser pulse of the same circular polarization is applied to said energized gas at a time .pi./.DELTA..omega. where .pi..DELTA. is the frequency splitting of the intermediate levels, to excite said excited intermediate levels to a higher energy level where an ionizing field ionizes the atoms or molecules in the higher level so that they may be separated from the un-ionized atoms or molecules of the other isotope.