Molecular laser isotope separation

Molecular hydrogen, 23 759 Molecular imprinting, 6 397 Molecular interactions, 25 103 Molecular interaction theories, 24 38 Molecular Laser Isotope Separation (MLIS) process, 25 416 417 Molecular level machine, 2 7 58 Molecularly imprinted plastics (MIPs) smart, 22 717)... 

The invention of lasers in the early 1960s made possible the laser isotope separation (LIS) approach. The first US research began in the 1960s at the Los Alamos National Laboratory (LANL) lasers were used to excite the U molecules in a UFj stream, a process they called molecular laser isotope separation (MLIS). Their work continued for a decade or more. 

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. 

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. 

Although this limit is not always reached. The same is true for the coherence of the radiation. Each of these properties can be exploited for particular chemical applications. The monochromaticity can be used to initiate a chemical reaction of particular molecules in a mixture. The laser isotope separation of C and C in natural abundance exploits the isotope shift of molecular vibrational frequencies. At 10-50 cm , the corresponding shift of IR absorption wavenumbers is large compared to the spectral width of the CO2 laser... 

Most methods of laser isotope separation are based on the selective excitation of the desired atomic or molecular isotope in the gas phase. Some possible ways for separating the excited species are depicted schematically in Fig. 10.15, where A and B may be atoms or molecules, such as radicals. If the selectively excited isotope Ai is irradiated by a second photon during the lifetime of the excited state, photoionization or photodissociation may take place if... 

The high extent of monochromaticity ensures a highly selective action on molecules, for instance, by inducing transitions only between one pair of rotational states. Since the laser line width is often smaller than the isotopic shift of molecular vibration-rotational frequencies, it appears possible to act selectively on iso-topicaUy substituted molecules. This is the basis of laser isotope separation [22, 77, 215, 267]. 

Most methods of laser isotope separation are based on selective excitation of the desired atomic or molecular isotope in the gas phase. Figure 14.2 depicts some possible ways of separating the excited species from the ground state isotopes. 

Another approach to laser isotope separation is offered by predissociation of laser-excited molecular isotopes into stable fragments. If the potential curve of the excited state of AB is intersected by a repulsive potential (Fig.14.3), the molecule may dissociate without absorbing a second photon. 

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]. 

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-... 

The structure of molecular complexes in their electronic ground state can be obtained from direct IR laser absorption spectroscopy in pulsed supersonic-slit jet expansions [9.47]. This allows one to follow the formation rate of clusters and complexes during the adiabatic expansion [9.48]. Selective photodissociation of van der Waals clusters by infrared lasers may be used for isotope separation [9.49]. 

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]. 

J.L. Lyman Laser-induced molecular dissociation. Applications in isotope separation and related processes, in [Ref.l0.1,p.417]... 

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. 

Letokhov, V. S. (1969). On the possibility of isotope separation by resonant atomic photoionization and molecular photodissociation with laser radiation. Report, of Lebedev Physical Institute, Nov. 1969. (Pubhshed in preprint No. 1 (1979) of the Institute of Spectroscopy, USSR Academy of Sciences, pp. I 54). 

The phenomenon of multiphoton dissociation finds a possible application in the separation of isotopes. For this purpose it is not only the high power of the laser that is important but the fact that it is highly monochromatic. This latter property makes it possible, in favourable circumstances, for the laser radiation to be absorbed selectively by a single isotopic molecular species. This species is then selectively dissociated resulting in isotopic enrichment both in the dissociation products and in the undissociated material. 

Another method, based on an old idea about radiation pressure, uses the local separation of different isotopes in atomic or molecular beams. If the laser beam which crosses the molecular beam at right angles is tuned to an absorption line of a defined isotope in a molecular beam containing an isotopic mixture, the recoil from the absorption of the laser photons results in a small additional transverse velocity component. This leads to a beam deflection for the absorbing molecules which enables the desired isotope to be collected in a separate collector 154g)... 

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