Some Examples of Isotope Enrichment

Photochemical schemes for isotope enrichment can be conveniently divided according to whether one, two, or more photons are used in each atomic or molecular separation. In the next few paragraphs, examples are given of each of these three categories. These examples have been chosen for their relevance to the main themes of this article—and indeed of this volume —and, in several cases, the dynamics of the systems are examined more fully later in the text. 

The potential utility of formaldehyde predissociation for isotope separation was pointed out independently by Yeung and Moore and Letokhov in 1972. Yeung and Moore recognized that there were predisso-ciative channels to both molecular and radical products. 

The early papers of Yeung and Moore and of Letokhov have spawned 

By observing the intensity of emission from vibrationally excited H Br formed in reaction (1.5b) as a function of the ratio of concentrations, [Br2]/[HI], Zaraga et were able to find the rate of the isotopic scrambling reaction 

The enrichment experiments which bear most closely upon the main topic of this chapter are those where a molecular isotomer is selectively excited and then reacts at a rate far in excess of the unexcited species. Hopes that vibrational excitation plus chemical reaction might provide viable enrichment schemes were raised by an experiment performed in 1970. Mixtures of CHgOH and CDgOD were irradiated with the output from an HF chemical laser in the presence of Bra. Product analysis suggested that the CHgOH, which absorbs the laser radiation, was selectively depleted. Unfortunately, a careful reexamination of this system has failed to discover any significant effect brought about by the laser, or by filtered blackbody radiation, and the conclusion was that the quantum yield for the photo-chemically induced reaction must be 10 per infrared photon absorbed. 

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