Isotope Separation with Lasers

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 ions can be separated from the neutrals by electric fields, which collect them into a Faraday cup. This technique has been used, for example, for the separation of atoms in the gas phase by resonant two-photon ionization with copper-vapor laser-pumped dye lasers at high repetition frequencies [1429]. Since the line density in the visible absorption spectrum of is very high, the lasers are crossed perpendicularly with a cold collimated beam of uranium atoms in order to reduce the line density and the absorption linewidth. 

In the case of molecular isotopes, the absorption of the second photon may also lead to photodissociation. The fragments R are often more reactive than the parent molecules and may react with properly added scavenger reactants S to form new compounds RS, which can be separated by chemical means. 

In favorable cases no second photon is necessary if reactants can be found that react with the excited isotopes M with a much larger probability than with the 

The isotope I Cl can be selectively excited at A = 605 nm by a cw dye laser. The excited molecules react in collisions with bromine benzene and form the unstable radical ClC6H5Br, which dissociates rapidly into C6H5 C1 -f Br. In a laboratory experiment, several milligrams of C6H5CI were produced during a two hour exposure. Enrichment factors K = Cl/ Cl oi K = 6 have been achieved [1426]. 

Even multiphoton dissociation of larger molecules such as SFg by CO2 lasers may be isotope-selective [15.28]. For the heavier molecule UF0 the necessary selectivity of the first step, excited at A = 16 /xm can only be reached in a collimated cold UF0 beam. The vibrationally excited UF0 isotope can then be ionized by a Xe-Cl-excimer laser at A = 308 nm [15.31]. The absolute amount of isotopes separated in this way is however very small [15.31b]. 

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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]. 

F.S. Becker, K.L. Kompa The practical and physical aspects of uranium isotope separation with lasers. Nuc. Technol. 58, 329 (1982)... 

Vanderleeden, J.C. Generalized concepts in large-scale laser isotope separation, with application to deuterium. J. Appl. Phys. 1980, 51, 1273. 

The volatility of U(OMe)6 has recently attracted attention for laser-induced uranium isotope separation with a CO2 laser. At 330°C, U(OMe)e has a vapor pressure of IVmTorr and A/f biimation = 96 13 kJmol and AS° b,i ation = 318 17 JK mol . From IR and Raman spectroscopic studies of the O and 0 labell methoxide, a good vibrational analysis has been performed, allowing the assignments of the U— O stretching frequencies 505.0cm" ... 

It may be surprising to find the most extensive application of collinear laser fast-beam spectroscopy in a field that a priori has little connection with the special features of this technique. Neither the Doppler shift nor the accessibility of ionic spectra plays a decisive role for the on-line experiments on radioactive isotopes from nuclear reactions. However, most of the problems encountered in the preparation of a sample of free atoms (cf. Part B, Chapter 17 by H.-J. Kluge) are solved by a combination of the fast-beam technique with the well-established concept of on-line isotope separation. The isotope separators (with ISOLDE at CERN as an outstanding example) provide the unstable species in the form of ion beams whose phase-space volume is well matched to the requirements of collinear spectroscopy. 

The recent discovery of "multiphoton dissociation" of polyatomic molecules, where molecules, such as SFg, can be dissociated by multiple absorption of infrared laser photons, has stimulated many theoretical [14.13] and experimental [14.14] investigations about the mechanism of this process. Since the first steps, namely the excitation of lower vibrational levels with moderate level density may be isotope selective, the multiphoton dissociation may turn out to become a cheap and efficient way of laser isotope separation. Infrared lasers, such as the CO2 laser, have a high conversion efficiency which makes CO2 laser photons inexpensive. For more detailed discussions of the various aspects of laser isotope separation see [14.15-17]. 

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. 

The existence of isotope shifts and of tunable lasers with narrow Hnewidth leads to the possibHity of separating isotopes with laser radiation (113,114). This can be of importance, because isotopicaHy selected materials are used for many purposes in research, medicine, and industry. In order to separate isotopes, one needs a molecule that contains the desired element and has an isotope shift in its absorption spectmm, plus a laser that can be tuned to the absorption of one of the isotopic constituents. Several means for separating isotopes are avaHable. The selected species may be ionized by absorption of several photons and removed by appHcation of an electric field, or photodissociated and removed by chemical means. 

A promising alternative is provided by Laser isotope separation . Because the ionization energies of and differ slightly, it is possible to ionize the former selectively by irradiating U vapour with laser beams precisely tuned to the appropriate wavelength. The ions can then be collected at a negative electrode. 

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 ... 

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]. 

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