Isotope separation by cw infrared laser enhanced reaction

Abstract

In laser isotope separation it is widely assumed that the rate of the chemical reaction which involves the laser excited reagents must exceed the interisotope energy transfer rate. This is shown to be an unnecessary constraint on the selection of reactions and experimental conditions. Using deactivation processes to compete with energy transfer between the isotopes, it is shown that isotope selectivity can be preserved even under the difficult conditions of cw single infrared photon excitation. The principle is demonstrated by showing bromine isotope selectivity in the radical chain chlorination of methyl bromide in a low pressure discharge–flow reactor intracavity to a CO2 laser. Thermal, VV, and chemical scrambling effects are shown to be unimportant with proper experimental design. Bromine isotope enrichment is limited to ≲5% by the small vibrational rate enhancement of the near-thermoneutral hydrogen abstraction. The temperature dependence of the enrichment is studied and the difficulty in assigning the increased reactivity to a particular vibrational mode is discussed. The energy cost for this particular isotopic enrichment is calculated (6 keV/product molecule) to demonstrate that although energy is wasted by the deactivation process, the flexibility this technique offers in terms of permitting the use of cw (large throughput) ir (efficient) lasers outweighs the above effect. The use of cw techniques allows changes in reactivity of less than 1% to be readily observed.