Abstract: A method and apparatus for laser isotope separation of atoms or molecules. The method for laser isotope separation comprises the irradiation of an atom or molecule to produce an adiabatic population inversion, terminating the radiation sufficiently abruptly to trap the atom or molecule in an excited state, and further radiating the atom or molecule for causing transitions out of the previously excited state for dissociating the molecule or for causing a chemical reaction to effect isotopic separation. The method comprises slowly increasing the intensity of the initial radiation from zero to a value sufficiently high to produce an adiabatic population inversion when the frequency is tuned near a single narrow resonance which includes the overlapping multiphoton resonances originating from either the rotational energy levels of the vibrational ground state of the molecule or the different hyperfine levels of the ground state of the atom for providing the adiabatic population inversion.