Isotope separation, using lasers

O. Atabek, M.Chrysos, R.Lefebvre, Isotope separation using intense laser fields, Phys. Rev. A 49 (1994) R8. 

However, it is important to recognize that the first few steps in the absorption process are selective. Owing to the low density of states in this region, only one molecular species (which has a transition resonant with the laser frequency) in a mixmre of other molecules will absorb and thus be selectively excited into the quasicontinuum and on to the dissociation limit. Indeed, it is possible to achieve isotope separation using IR multiple-photon excitation for example, can be selec-... 

Laser isotope separation techniques Laser-based isotope enrichment techniques deploy selective photo-excitation principles to excite a particular isotope as an atom or molecule (Rao 2003). Each device consists of three parts the laser system, the optical system, and the separation module. These methods include the atomic vapor laser isotope separation (AVLIS) that uses a fine-tuned laser beam to selectively ionize vapors of atomic the molecular laser isotope separation (MLIS), and separation of isotopes by laser excitation (SD EX) that use a laser to selectively dissociate or excite molecules. 

In the previous chapter we have seen how tunable lasers can be used in a multitude of ways to gain basic information on atomic and molecular systems. Thus, the laser has had a considerable impact on basic research, and its utility within the applied spectroscopic field is not smaller. We shall here discuss some applications of considerable interest. Previously, we have mainly chosen atomic spectroscopic examples rather than molecular ones, but in this chapter we shall mainly discuss applied molecular spectroscopy. First we will describe diagnostics of combustion processes and then discuss atmospheric monitoring by laser techniques. Different aspects of laser-induced fluorescence in liquids and solids will be considered with examples from the environmental, industrial and medical fields. We will also describe laser-induced chemical processes and isotope separation with lasers. Finally, spectroscopic aspects of lasers in medicine will be discussed. Applied aspects of laser spectroscopy have been covered in [10.1,2]. 

Uranium isotope separation using a molecular approach is based on selective multi-photon dissocation of UPg. The relevant vibrational isotope shift is 0.6 cm in the primary vibrational transition at 628 cm (16 pm). In the development of the technique, experiments on SFg have been very important. The conditions are inucli more favourable for SFe than for UFg- The isotope shift is 17cm between and in the IR active vibrational mode that involves asymmetric stretdiing of two S-F bonds. The spectrum has a typical P, Q and R branch structure and the whole region of absorption for the rotational level popnlation distribution that is obtained at room temperature is 15 cm . Thus, the isotopic molecules are spectroscopically totally separated. Furthermore, the vibrational transition in SFg well matches the emission of a free-ruiniing pulsed CO2 laser. 

Due to the very high intensity of the laser beams and their coherent nature they may be used in a variety of ways where controlled energy is required. Lasers are used commercially for excitation with a specific energy, e.g. in Raman spectroscopy or isotope separation. 

Although this limit is not always reaehed. The same is true for the eoherenee of the radiation. Eaeh of these properties ean be exploited for partieular ehemieal applieations. The monoeliromatieity ean be used to initiate a ehemieal reaetion of partieular moleeules in a mixture. The laser isotope separation of and in nafriral abimdanee exploits the isotope shift of moleeular vibrational frequeneies. At 10-50 em, the eorresponding shift of IR absorption wavenumbers is large eompared to the speetral width of the CO2 laser... 

LLNL AVLIS Laser. The first WFS measurements using a Na LGS were performed at LLNL (Max et al., 1994 Avicola et al., 1994). These experiments utilized an 1100 W dye laser, developed for atomic vapor laser isotope separation (AVLIS). The wavefront was better than 0.03 wave rms. The dye laser was pumped by 1500 W copper vapor lasers. They are not well suited as a pump for LGSs because of their 26 kHz pulse rate and 32 ns pulse length. The peak intensity at the Na layer, with an atmospheric transmission of 0.6 and a spot diameter of 2.0 m, is 25 W/cm, 4x the saturation. The laser linewidth and shape were tailored to match the D2 line. The power was varied from 7 to 1100 W on Na layer to study saturation. The spot size was measured to be 7 arcsec FWHM at 1100 W. It reduced to 4.6 arcsec after accounting for satura-... 

Experiments on the sky. Two experiments have been carried out at the sky, using two laser installations built for the American and French programmes for Uranium isotope separation, respectively AVLIS at the Lawrence Livermore Nat l Lab (California) in 1996 and SILVA at CEA/Pierrelatte (Southern France) in 1999. The average power was high pa 2 x 175 W, with a pulse repetition rate of 12.9 and 4.3 kHz, a pulse width of 40 ns and a spectral width of 1 and 3 GHz. Polarization was linear. The return flux was < 5 10 photons/m /s (Foy et al., 2000). Thus incoherent two-photon resonant absorption works, with a behavior consistent with models. But we do need lower powers at observatories ... 

Uranium enrichment using LIS has been exhaustively studied and the conceptual outlines of two different methods can be found in the open literature. These methods are multi-photon dissociation of UF6 (SILEX, or Separation of Isotopes by Laser Excitation) and laser excitation of monatomic uranium vapor (Atomic Vapor Laser Isotope Separation, or AVLIS). Following an enormous investment, AVLIS was used by the United States DOE in the 1980s and early 1990s, but due to the present oversupply of separated uranium, the plant has been shut down. 

Other uses of lasers include eye surgery on detached retinas, spot welding, holography, isotope separation, accurate determination of the moon s orbit by reflection of laser light off a reflector placed on the moon s surface, and laser-guided bombs and missiles. Possible future uses include terrestrial and extraterrestrial communication, applications to computers, and production of the high temperatures needed for controlled nuclear-fusion reactions. 

A similar technique has been used by Zare et al. (261, 643) for chlorine isotope separation. Isotopic mixtures of iodine monochloride (l35CI, lJ7CI) are irradiated in the presence of dibromoethylene by a laser line at 6053 A which selectively excites I37C1. An adjacent vibrational band of I35C1 is about 15 A away. The excited I37C1 reacts with added 1,2-dibromoethylene lo form the product f/wi.v-ClHC=CHCI enriched in 37C1. At this wavelength no photodissociation of ICI takes place. See p. 191. 

Prior to about 1955 much of the nuclear information was obtained from application of atomic physics. The nuclear spin, nuclear magnetic and electric moments and changes in mean-squared charge radii are derived from measurement of the atomic hyperfine structure (hfs) and Isotope Shift (IS) and are obtained in a nuclear model independent way. With the development of the tunable dye laser and its use with the online isotope separator this field has been rejuvenated. The scheme of collinear laser/fast-beam spectroscopy [KAU76] promised to be useful for a wide variety of elements, thus UNISOR began in 1980 to develop this type of facility. The present paper describes some of the first results from the UNISOR laser facility. 

Selective excitation of wavepackets with ultrashort broadband laser pulses is of fundamental importance for a variety of processes, such as the coherent control of photochemical reactions [36-39] or isotope separation [40--42]. It can also be used to actively control the molecular dynamics in a dissipative environment if the excitation process is much faster than relaxation. For practical applications it is desirable to establish an efficient method that allows one to increase the target product yield by using short laser pulses of moderate intensity before relaxation occurs [38]. 

The separation of chemical isotopes is based on small differences in their physical and chemical properties. For the lower-mass isotopes, chemical exchange, distillation, and electrolysis have been used. For the higher-mass isotopes, techniques based on mass have been used, including gaseous diffusion, centrifugation, thermal diffusion, and ion activation.29,30 A newer method uses lasers that produce coherent light tuned to the specific wavelength of a vibration bond related to the desired isotope in an atom or molecule. This technique is still under development but... 

The dramatic growth occurring over the past few years in laser chemistry and laser isotope separation has refocused interests upon dissociative processes in molecules. Collectively, these interests are traceable to the pragmatic goals of producing appreciable populations of selected atomic or molecular states having useful reactive properties or isotopic content. From this perspective, it is natural that photodissociation of some parent molecule would appear to be the ideal means for obtaining a desired product. 

Isotope Separation.— The IRMPE of CF2HCI and of 1% CF2DCI in CF2HCI has been investigated using several P- and R-branch lines of a CO2 laser by Kutschke et Optical selectivities obtained from these studies indicated... 

A method for Li isotope separation has been described by Arisawa et which makes use of the increased reactivity of a laser-excited beam of Li atoms with a beam of CHClFj. Isotope enrichment in the product LiF was observed, but no selectivity in LiCl was obtained. Balia and Heicklen have studied the photo-oxidation of CHjSH and CH3S2CH3 in the presence of NO. Chemiluminescent emission in Mg-N20-C0 flames has been studied by Michels and Meinzer. Inoue ef measured electron-impact lumines-... 

Bradley reported that homoleptic uranium hexakis(alkoxide) complexes coordinated by secondary and tertiary alkoxides (U(OR)6 R = Pr , Bu , Bu ) were produced from thermal disproportionation of U0(0R)4 vide suprd) U(OMe)6 was initially prepared from oxidation of U (OMe)5 in the presence of benzoyl peroxide. Interest in a more convenient synthetic route to U(OMe)6 was stimulated by its potential use in uranium isotope separation, which can be achieved with a CO2 laser. Facile syntheses of U(OMe)6 were reported by different groups (see Equations (48) to (51)) ... 

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