Multiphoton dissociation and laser separation of isotopes

The process of dissociation by fhe absorption of infrared photons clearly involves fhe simulfaneous absorption of many photons - of fhe order of 30, depending on fhe dissociation energy and fhe photon energy - and is called multiphofon dissociation. 

Based on the Wilson numbering for benzene (see the bibliography for Chapter 6). 

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. 

One of the first applications of this technique was to the enrichment of and B isotopes, present as 18.7 and 81.3 per cent, respectively, in natural abundance. Boron trichloride, BCI3, dissociates when irradiated with a pulsed CO2 laser in the 3g vibrational band at 958 cm (vj is an e vibration of the planar, D j, molecule). One of the products of dissociation was detected by reaction with O2 to form BO which then produced chemiluminescence (emission of radiation as a result of energy gained by chemical reaction) in the visible region due to A U — fluorescence. Irradiation in the 3g band of BCls or BCI3 resulted in °BO or BO chemiluminescence. The fluorescence of °BO is easily resolved from that of BO. 

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