Abstract: Method and apparatus for inducing isotopically selective photoexcitation of atoms in at least first and second energy steps. The total energy change accomplished by the two energy steps is provided with isotopic selectivity at precise resonance for the two steps. In an isotope separation system in which an environment of particles of plural isotope types is created, isotopically selective photoexcitation from laser radiation is shown to produce a great improvement in excitation efficiency where the radiation is controlled in the aggregate of plural photons to achieve precise resonance over the plurality of the degenerate energy steps. Even with multimode excitation, precise resonance is only achieved for isolated atoms, and a chirp or frequency sweep is applied to one of several excitation frequencies to insure precise resonance at least once for each atom.