The present invention pertains generally to isotope separation and in particular to laser-induced separations of hydrogen isotopes.
The objective of laser-induced separations of isotopes is to selectively transform molecules of one isotope into an enriched chemically distinct species which is capable of being chemically separated by subsequent processing and hence enriched. Generally, this type of isotope separation involves a preferential vibrational excitation of molecules of the desired isotope, followed by a chemical reaction, a uv photodissociation, or photoionization.
Even if molecules containing a certain isotope absorb energy preferentially, isotopic specificity can be destroyed by intermolecular VV energy exchanges between the various isotopically substituted species or by bulk heating. Interisotope VV transfer is near-resonant and therefore very fast, typically on the order of ten gas kinetic collisions. For a given molecule, it is very difficult to find a reaction which proceeds at a rate comparable to a VV transfer, simultaneously shows appreciable vibrational enhancement and yields a product which does not undergo a rapid chemical isotope exchange with the reagents. Bulk heating, due to VT relaxation of the excited species, increases the rate of the nonselective thermal reaction and therefore can completely mask the vibrational component of the reactivity change.
Besides isotopic specificity, an isotope separation must have several additional characteristics in order for it to have any commercial potential. Due to energy costs, isotopic spcificity occurring at excitations among the lowest two or three vibrational levels is most desirable. The separation must also be pressure scalable to be commercially viable. In other words it must be able to operate over a wide range of pressure, particularly pressures above those typically employed in the resarch laboratory.
Laser-induced separations involving spontaneous photodissociations due to vibrational excitation of low-lying vibrational levels are few. Large fractionation ratios are disclosed for the decomposition of D.sub.3 BPF.sub.3 and H.sub.3 BPF.sub.3 in K--R Chien and S. H. Bauer, J. Phys. Chem. 80(13), 1405 (1976). This method utilizes a direct absorption of photons by a costly reagent and thus can only proceed if a laser can operate at the absorption band and if no excessive bulk heating of the reactant occurs.
The separation techniques disclosed in U.S. Pat. No. 4,097,384 issued to Coleman et al on June 27, 1978, includes dissociating an uranium ligand as well as the more common scheme of a subsequent preferential reaction of the selectively excited molecules with another reactant. This method also relies on a direct absorption of photons to obtain a vibrational excitation of the molecules and has the same problems previously discussed. The necessity of having a laser operate in the fundamental absorption band of the molecule being excited is attempted to be solved by relying on absorptions at the overtones. These absorptions are relatively small and thus scalability to larger systems is difficult.
The separation method in S. W. Mayer, M. A. Kwok, et al, App. Phys, Lett. 17, 516 (1970) is another direct absorption method. An H:D separation is achieved through an isotopically specific reaction in a CH.sub.3 OH:CH.sub.3 OD:Br.sub.2 gas-phase mixture at a pressure of about 100 torr which has been excited with a 90 w cw HF laser. All attempts to reproduce the results have failed, due probably to bulk heating of the gas mixture, VV transfer, and chemical isotopic scrambling between reagents and products.
As was stated previously, a major cause for failures of isotope separation attempts is the rapid rate of intermolecular VV energy exchanges between isotopically substituted species exceeding the rate of the chemical differentiation step. This whole question has been left unanswered in previous research work. It has been pointed out that if the total rate of deactivation of the excited vibration is comparable to or faster than the rate of interisotopic VV exchange, isotopic selectivity on a CW basis is preserved as well as establishing pressure scalability. This concept has been termed the competing-deactivation technique. In Manuccia et al., J. Chem. Phys. 68(5), Mar. 1, 1978, this technique is used to enrich .sup.79 Br or .sup.81 Br in the products of the radical chain reaction of chlorine atoms with natural isotopic-abundance methyl bromide by exciting the respective CH.sub.3 Br in a low-pressure, discharge flow reactor intracavity to a CO.sub.2 laser.
A deuterium separation by the competing-deactivation technique is disclosed in Hsu et al Advances in Laser Chemistry, ed. by A. H. Zewail, Springer Series in Chemical Physics, p. 88-92, and in Hsu et al Appl. Phys. Lett. 33(11), p. 915-17, (Dec. 1, 1978), A CW CO.sub.2 laser is used to vibrationally excite CH.sub.2 D.sub.2 in a mixture of CH.sub.2 D.sub.2 and CH.sub.4 while an intentional VT deactivation by argon atoms and the reactor walls competes with interisotope VV transfer to produce a gas sample in which the CH.sub.2 D.sub.2 is excited and the CH.sub.4 remains less excited on a steady state basis. A reaction of this gas mixture with chlorine atoms and molecules forms a stable product, deuterated methyl chloride, enriched in deuterium by up to 72%. This method can be considered technically important because it is the most energy efficient laser method for deuterium to date. The projected energy efficiency, however, is below that of the current H.sub.2 S/H.sub.2 O process because of the high energy costs of pumping, refrigeration and reactant regeneration imposed by use of the thermoneutral reaction.