Energy level effects during multiphoton dissociation and the laser separation of closely spaced isotopes

Abstract

A novel approach for enhancing the selectivity of the desired isotope in the molecular laser isotope separation (MLIS) process is presented. The scheme consists of simultaneously applying two laser beams with frequencies corresponding to those between the ground and the first energy excitation level and the ground and the second energy excitation level, respectively. Practical relations on the properties of the spherical-top molecules are derived and a semiclassical analysis of the electromagnetic interaction within the limits of the experimental conditions applied in actual MLIS experiments shows that the selectivity, defined as the ratio of the absorption cross sections of the two isotopes, increases by a factor of 10–20 times in the case of the uranium isotopes. In addition, it is demonstrated that during the multiphoton absorption process energy-level splittings due to induced magnetic dipoles and induced electric quadrupoles are by no means negligible. They become significant during multiphoton processes where two or more photons are lost during the interaction process. At high pumping powers they become dominant and inhibit selectivity. They cancel out during interaction processes where there is no change in the total number of photons, such as scattering. These effects can be avoided by applying the laser beams to the molecular gas in arrangements which in principle are equivalent to a Mach–Zehnder interferometer with the molecules substituted for the reuniting beam splitter. Moreover, the induced electric quadrupoles (E2) are fully exploited. The application of the results and the concepts described herein can render the MLIS process the most economic and practical method for the commercial separation of the uranium isotopes.