Isotope-selective photodissociation

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). 

Isotope-selective photodissociation of gaseous carbon monoxide is a well-known process in molecular clouds. This process may have been important in the early solar system, with the nascent Sun as the source of ultraviolet radiation. The consequent self-shielding gives rise to the non-mass-dependent oxygen isotope variations observed in primitive meteorites. This model implies that the solar oxygen isotope ratios, and are about 5% smaller than those ratios in... 

Fig. 11.1 Various schemes for isotopically selective photodissociation of molecules via vibrational states, and the first experiments (molecules, isotopes, and years).

Fig. 11.2 (a) Schematic diagram of the IR-UV isotope-selective photodissociation of molecules AB in a mixture with molecules AB of different isotopic composition via an intermediate vibrational state (for example, u = 2) (b) change of absorption spectrum. (Modified from Letokhov 1973a.)... 

In addition to SO2 self-shielding many other possible sources of S-MIF can be identified. The model A S/results for the case of a I0W-O2 atmosphere (e.g.. Figure 5.7b) are in qualitative but not quantitative agreement with the ancient rock record. Elemental S, derived from S(D, is predicted here to have > 0 and < 0, which is consistent with observations of most pyrites [4], but the magnitude of the A S/A S ratio is about a factor of 2 to 3 too high ( -2.5 vs. 0.9). This is a significant discrepancy, and may indicate that MI processes in addition to SO2 photodissociation are at work. One such MI process almost certain to be important in a I0W-O2 atmosphere is SO photodissociation. Isotope-selective photolysis will occur in SO at wavelengths 190-230 nm, but rotationally-resolved spectra, either laboratory or synthetic, are needed to estimate the MI effect. In addition to S-MIF due to SO photolysis, SO2 photoexcitation ( 280-330 nm) and SO3 photolysis [18] must also be considered as possible contributors to S-MIF in the ancient atmosphere. S-MIF due to these photo-processes will be considered in future work. 

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... 

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]. 

Probably the first suggestion for utilizing the properties of laser light (the high intensity and short duration of radiation pulses) was (Letokhov 1969) to use the vibrationally mediated photodissociation of molecules via an excited repulsive electronic state with noncoherent isotope-selective saturation of the vibrational transition (Fig. 11.2). The isotope-selective two-step photodissociation of molecules consists of pulsed isotope-selective excitation of a vibrational state in the molecules by IR laser radiation and subsequent pulsed photodissociation of the vibrationally excited molecules via an excited electronic state by a UV pulse (Fig. 11.2(a)) before the isotope selectivity of the excitation is lost in collisions. Selective two-step photodissociation of molecules is possible if their excitation is accompanied by a shift of their continuous-wave electronic photoabsorption band. In that case, the molecules of the desired isotopic composition, selectively excited by a laser pulse of frequency uji, can be photodissociated by a second laser pulse of frequency uj2 selected to fall within the region of the shift where the ratio between the absorption coefficients of the excited and unexcited molecules is a maximum (Fig. 11.2(b)). 

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 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. 

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. 

Since the invention of the tunable laser chemists have dreamed of using its characteristic high intensity and spectral purity to control the selectivity of chemical reactions. When such selectivity can be achieved via use of spectral resolution, for example, photodissociation of different isotopic variants of the same chemical species, the desired control of product formation has been demonstrated. On the other hand, when such selectivity is sought via concentration of energy in particular bonds, rapid intramolecular energy transfer has prevented, for the examples studied to date, the desired control of product formation. 

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