* ince the advent of lasers, these unique sources of highly intense and nearly monochromatic radi- ation have been proposed as tools sto induce or catalyze chemical reactions. Of Based on recent discoveries in science and advances all the reactions investigated, laser isotope separation has received the most attention in engineering, the Los Alamos molecular worldwide and may be the fust major com- mercial application of lasers to chemistry. Laser isotope separation was first demon- laser isotope separation process appears to be an strated nearly a decade ago for boron and to date has been applied on a laboratory scale 1 economical method for uranium enrichment. to many elements throughout the periodic table. But the goal is to find laser processes that are more economical than conventional by Reed J. Jensen, O’Dean P. Judd, and J. Man Sullivan separation techniques. Los Alamos re- searchers have developed a practical process for separating sulfm isotopes based on laser irradiation of sulfur hexatluoride molecules, and the Soviets have developed commercial- ly applicable laser processes for separating both sulfur and carbon isotopes. However, the primary motivation behind the generous funding of this field is the 2 LOS ALAMOS SCIENCE Separating Isotopes With Lasers promise of an economical method for pro- of uranium-235 to that required of reactor thousand chambers are needed to increase ducing bulk quantities of enriched uranium, fuel, the two isotopes must be sorted accord- the concentration of uranium-235 to the fuel the fuel of nuclear reactors. Here, success ing to some difference in their chemical or assay of 3.2 per cent required for light-water has been much harder to achieve. But the physical properties. But the electronic and reactors. Gaseous diffusion thus requires a difficulties encountered have been beneficial therefore the chemical properties of the two very large and expensive facility and, in the larger perspective, having stimulated isotopes are so nearly identical that chemical moreover, consumes large amounts of elec- fundamental scientific advances that are processing is difficult and inefficient. Con- trical energy. strongly influencing the broad field of laser ventional methods for separating isotopes of chemistry. Among them are the discovery of Other separation techniques based on uranium, as well as those of other elements, mass differences include the gas centrifuge, multiple-photon processes, a revolution in rely instead on physical processes that are infrared spectroscopy of heavy molecules, an multiple distillation, and electromagnetic sep- affected by the small differences in the aration. Of these, the gas centrifuge is being increased understanding of molecular elec- masses of the different isotopes. tronic structure and of condensation proc- explored as an alternative to gaseous dif- The gaseous diffusion method which cur- esses in cooled gases, the development of fusion (see sidebar “Economic Perspective new, high-intensity, tunable laser systems, rently produces most of the enriched for Uranium Enrichment”). and practical methods for producing gas uranium for nuclear reactors, consists of What may prove to be more economical is flows at low temperatures. These advances passing gaseous uranium hexafluoride a separation process driven by lasers. The have also contributed to major progress in molecules (UF6) through a series of cham- idea is quite simple. Since atoms or the Laboratory’s molecular laser isotope bers separated by porous barriers. The light- molecules containing different isotopes have separation process for uranium. er molecules, those containing uranium-235, slightly different energy levels, they have Natural uranium is a mixture of isotopes diffuse through the barriers slightly faster. So slightly different absorption spectra-that is, and contains 99.3 per cent uranium-238 and in each successive chamber the concentra- they absorb radiation with different frequen- only 0.7 per cent of the fissile isotope tion of uranium-235 relative to that of cies (Fig. 1). Consequently, radiation of a uranium-235. To increase the concentration uranium-238 increases slightly. More than a particular frequency can selectively excite an LOS ALAMOS SCIENCE 3 Dissociation Limit Wavenurnber (c) Fig. 1. The shift in vibrational energy levels of one molecular shown in more detail. Each vibrational state is split into many isotopic species relative to another shown in (a) is reflected in rotational states labeled by J, the number of rotational angular (c) as a shift in the infrared absorption spectra. (a) Within the momentum quanta of the state. A t room temperature molecules ground electronic state of a molecule are many vibrational typically populate rotational states with high J values. During states resulting from oscillatory motion of the nuclei about their equilibrium positions. Shown here schematically are the restricted to —1, O, or +1. Such allowed transitions are energy levels for a vibrational mode of UFh known as the vl denoted as P-,Q-, and R-branch transitions, respectively. (c) 235 238 mode. The energy levels are labeled by v, the number of The infrared absorption bands of UFb and UFe from 620 vibrational energy quanta of the state. The arrows represent to 630 reciprocal centimeters include transitions from the absorption of infrared photons that raise a molecule from the ground state to the first excited state of the v, vibrational ground state to the first vibrational state. The different lengths mode. Absorption that excite the VB mode occur over a broad of the arrows for the two isotopic species represent the different band of frequencies because molecules in the ground state photon energies, or frequencies, needed to excite the two occupy many rotational states and the molecules in each isotopic species. The difference, although small (less than 1.25 rotational state can undergo P-, Q-, or R-branch transitions to -4 X 10 electron volt, or 1 reciprocal centimeter, for UF6, the first excited vibrational state. The absorption band of 235 allows selective excitation by nearly monochromatic laser UF6, is shifted slightly to higher frequencies relative to that 238 light. (b) One of the vibrational transitions indicated in (a) is of UF6. 4 LOS ALAMOS SCIENCE Separating Isotopes With Lasers atom or molecule containing one isotope to a Alamos and at many other research centers covered only a small number of wavelengths. higher energy level and leave other isotopic around the world since the early 1970s when Monochromatic sources at other species undisturbed. Then, depending on the high-intensity tunable lasers became avail- wavelengths were created from conventional type of excitation, the selectively excited able. However, isotope separation based on white light sources (for example, with filters species can be separated from the others by selective photoexcitation of atoms and or gratings), but their intensity was even conventional physical or chemical methods. molecules is not a new idea. In fact, lower. For selective excitation to be practical as a photochemical separation was attempted High-intensity tunable lasers have re- separation technique, it must produce a large with conventional radiation sources long moved many of the limitations of the early change in some chemical or physical proper- before the invention of lasers. In 1922 efforts experiments. Lasers can be tuned to match ty of the excited species. One possibility is to were made to separate the two naturally any absorption feature that shows a distinct excite a molecule to such a high energy level occurring chlorine isotopes by irradiating isotope shift. In particular, high-intensity that its chemical reactivity increases substan- them with white light that had been filtered infrared lasers can selectively excite the tially. The molecule can then react with through a cell containing only the more isotonically distinct vibrational levels of another chemical species, and the product abundant chlorine isotope. These experi- molecules. Because of its high containing the desired isotope can be sepa- ments were unsuccessful. About ten years monochromaticity, laser light can excite a rated from the mixture by conventional tech- later Stanislaw Mrozowski suggested that desired species with reasonable selectivity niques. This type of bimolecular process has mercury isotopes might be separated by even when absorption features of other many applications in selective photochemis- selective excitation with the 253.7-nanometer isotopic species partially overlap those of the try. resonance line of a mercury arc lamp and desired isotopic species. Thus, both the However, the more widely studied laser subsequent reaction with oxygen. This sepa- tunability and high monochromaticity of the isotope separation techniques involve only ration was achieved experimentally by Kurt laser are crucial for selective excitation. photons and a single atomic or molecular Zuber in 1935. In the early ’40s Harold Urey Other properties of laser light contribute species. For example, the atomic vapor proc- proposed a photochemical method for sepa- to the efficiency of selective excitation. First, ess under development at Lawrence Liver- rating uranium isotopes, but his proposal lost since laser light has a high degree of spatial more National Laboratory uses selective in competition with the gaseous diffusion and temporal coherence, a laser beam can photoionization to separate uranium technique. After the war, an enlarged effort propagate over long distances