Isotope selective dissociation

If the first step of the multiphoton excitation can be chosen isotope selectively so that a wanted isotopomer has a larger absorption probability than the other isotopomers of the molecular species, selective dissociation may be achieved [7.68], which can be used for isotope separation by chemical reactions with the isotope-selective dissociation products (Sect. 15.2). 

The photoselective laser-induced excitation and dissociation of molecules have been described in detail in a munber of monographs and reviews (Jortner et al. 1981 Letokhov 1983 Bagratashvili et al. 1985f> Letokhov 1989 Quack 1998). Therefore, we shall present below only brief information about the progress made in this field. The main schemes for isotope-selective dissociation of molecules via vibrational states are shown in Fig. 11.1. Accordingly, the problem of photoselective laser control of molecules will be considered below in consecutive order, starting with the simplest scheme of Fig. 11.1(a) and ending with that of Fig. 11.1(c). 

The first experiment on isotope-selective dissociation was carried out with the and molecules, which could be selectively excited by C02-laser radiation... 

Petrov, A. K., Chesnokov, E. N., Gorelik, S. R., Straub, K. D., Szarmes, E. B., and Madey, J. M. J. (1997). Multiphoton isotope-selective dissociation of formic acid molecules under action of a free electron laser. Journal of Physical Chemistry, 101, 7200-7207. 

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. 

Since the dissociation of the abundant H2 and CO molecules result from photoabsorption at discrete wavelengths, isotopically selective photodissociation based on self-shielding is possible. Two conditions are required for this (1) dissociation via line absorption for each isotopically substituted molecule, and (2) differential photolysis that depends upon the isotopic abundances. Self-shielding occurs when the spectral lines leading to dissociation of the major isotopic species optically saturate, while the other residual lines relevant for dissociation of the minor isotopes remain transparent. As a consequence, such photolysis depends on nucleic abundance rather than the mass of a molecule see Fig. 4.4 (Langer 1977 Thiemens Heidenreich 1983). 

Measurements have been made of the u.v. dissociation of UF at 260—320 nm and of the relaxation dynamics of UF6 following u.v. excitation. Comparisons were made with dissociation data, and a previously known simple kinetic model has been improved. Results have also been presented on the cross-sections involved in the irradiation of UF simultaneously with i.r. and u.v. lasers as functions of wavelength, fluence, and temperature. A model is advanced to express the isotope selectivity as a function of u.v. 

Isotope-selective IR multiphoton photodissociation of CF3I has been examined as a means of achieving C enrichment, and about 400-fold enrichment has been reported following a single laser pulse in a short-path gas dynamic flow system. " Multiphoton IR dissociation of CHCIF2 has been shown to... 

This chapter is concerned with experimental investigations of the dynamics of the dissociation of polyatomic neutral molecules carried out by the technique of laser Doppler spectroscopy, in bulk and under crossed-beam condition. Photodissociation is a basic process in the interaction of light with molecules, of interest in itself as an elementary molecular process and also with respect to a variety of applications in different fields. The interest has increased considerably in recent years, first, because the experimental investigation of photodissociation is rapidly advancing by the use of the laser, and second, because the laser makes possible to achieve photodissociation, state, and isotope selectively, by new excitation mechanisms. These are, aside from the common one-photon absorption, stepwise... 

Photofragmentations of Organosilicon Compounds. - There have been several studies of the infrared multiphoton decomposition of silanes. Isotope-selective IR multiphoton dissociation of difluorosilane was achieved with 977.2... 

Heck, A.J.R. Derrick, P.J. Ultrahigh mass accuracy in isotope selective collision-induced dissociation using correlated sweep excitation and sustained off resonance iradiation A Fourier transform mass spectrometry case study on the [M + 2H]2-t ion of bradykinin. Anal. Chem. 1997, 69, 3603-3607. 

Even multiphoton dissociation of larger molecules such as SFe by CO2 lasers may be isotope selective [1424], For the heavier molecule UFe the necessary selectivity of the first step, excited at A, = 16 pm, can only be reached in a collimated cold UFe beam. The vibrationally excited UFg isotope can then be ionized by a XeCl ex-cimer laser at A = 308 nm [1427a], The absolute amount of isotopes separated in this way is, however, very small [1427b],... 

Rate constants for the oxidative addition to the 14-electron intermediate were obtained directly from flash photolysis experiments. Flash photolysis of RhCl(CO)(PPh3)j causes selective dissociation of CO to form the 14-electron intermediate RhCl(PPh3)j or a derivative of it containing coordinated benzene (Scheme 6.2). Oxidative addition of dihydrogen to this intermediate was observed directly by timeresolved UV-vis spectroscopy and was shown to be fast k = 1.0 x 10 M" s and AG = 10 kcal/mol). A small kinetic isotope effect was measured, k /k = 1.4, and this value is consistent with an early, three-centered transition state in which little H-H bond breaking has occurred. 

Even multiphoton dissociation of larger molecules such as SF6 by CO2 lasers may be isotope selective [15.52]. For the heavier molecule UF6 the necessary selectivity of the first step, excited at A = I6 xm, can only be reached... 

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 isotopic selectivity of IR MP dissociation was first shown in experiments with the °BCl3 and BCls molecules (Ambartzumian et al. 1974). In this case, of isotope shift in the frequency of the 10 /xm vibration being excited was 39 cm . A great number of subsequent experiments with different molecules showed that the case of BCI3 molecule was not unique and that the selectivity of MP dissociation is a characteristic property of it. In experiments (Horsley et al. 1980) on the separation of the uranium isotopes and isotopic selectivity of dissociation was observed when the... 

Many experiments on the isotopic selectivity of MP molecular dissociation have thus far been performed. These experiments have covered many isotopes, from light... 

There were also some remarkable meetings abroad during various scientific schools and conferences. I recall with pleasure my talks with Professor Ali Javan at the 1975 Les Houches School on Laser Spectroscopy (Fig. 14.12), and with Professor A. Siegman during my visit to Stanford University (Fig. 14.13), where we discussed animatedly the effect of isotope-selective multiple-photon dissociation of polyatomic molecules by IR laser pulses. At regular international conferences on laser spectroscopy and atomic... 

Fig. 14.13 Professor Anthony Siegman discussing with the author the mechanism of isotope-selective multiple-photon dissociation of SFe molecules under the effect of high-power IR-laser pulses at his office at Stanford University (in the late 1970s).

Boyarkin, O. V., Kovalczyk, M., and Rizzo, T. R. (2003). Collisionally enhanced isotopical selectivity in multiphoton dissociation of vibrationally excited CF3H. Journal of Chemical Physics, 118, 93-103. 

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