Abstract: A method of isotope-selectively exciting gaseous or vaporous uranium hexafluoride molecules by subjecting them to the action of a monochromatic iodine laser beam, the frequency of which can be adjusted and tuned to an absorption band of the molecules to be excited, the laser beam being scattered by liquid and/or solid nitrogen to obtain a triple Raman-scattering. In a preferred embodiment, the laser has an emission frequency of 7600 to 7610 cm.sup.-1 and the tuning is effected by means of a magnetic field.An apparatus suitable for carrying out such a method comprises a high-performance iodine laser and an optical resonator into which the emission beam or pulses of the laser are focused, one or more Dewar vessels filled with liquid or solid nitrogen being located within the optical resonator. In a preferred embodiment, the laser beam tube is located between the poles of an electromagnet.