Determination of two-photon-excitation cross section for molecular isotope separation

► Two-photon excitation in methyl iodide might be useful for isotopic laser separation. ► The enrichment technique is based on excitation of a v3 hot band. ► The cross section of 6s Rydberg [3] was measured by a modified pump-probe technique. ► The enrichment factor of the separation was estimated. ► The method is promising for isotopic enrichment of molecules.