Appl. Phys. 24,143-146 (1981) Applied Physics @ Springer-Verlag t981 Improved Isotope Separation of Trifluoromethyl Halides" Scavenging of CF 3 with HI in Laser Dissociation C. N. Plum and P. L. Houston* Department of Chemistry, Cornell University, Ithaca, NY 14853, USA Received 19 May 1980/Accepted 2 October 1980 Abstract. The efficiency of 13 C isotope separation by multiphoton dissociation using a CO 2 laser has been shown to increase by ~ 33 % upon addition of small amounts of HI to the starting mixture. The reason for this increase is that the CF 3 radicals are scavenged by HI more rapidly than they recombine with free iodine atoms to reform the starting material. For Pcv3 =0.05 Torr and for 0.5 J/cm 2 roughly 65 % of the CF 3 and I radicals reform the starting material in the absence of HI. The results of this study indicate that at -80 ~ the rate constants for CF 3 + CF 3-+C2F 6 and CF 3 + I--, CF3I are about equal and are roughly 8 times higher than that for CF 3 +HI~CF3H+ I. PACS: 33.80K, 82.20P, 82.30 Laser dissociation of trifluoromethyl halides has been shown by a number of investigators to be a potentially important route to carbon isotope separation. Although the details of the selectivity and of the dissociation dynamics vary according to whether CF3I [1-5], CF3Br [2,5,6], or CFaC1 I-2] is used as a starting compound, all of these carbon separation schemes use recombination of the CF 3 radicals via reaction (1) as their final step. kl CF3 + CF3 ~ CzF 6 9 (1) Thus, the efficiency of separation for any of these schemes depends on how effectively the product frag- ments are removed by reaction (1) in competition with other possible processes. These might include CF 3 -I- CF3 X~products , CF 3 +Xz-+CF3X+ X, and (2) (3) C F 3 -t- X k4 ) CF3X. (4) Occurrence of reaction (2) is unlikely on energetic grounds. Reaction (3) can take place only after X 2 has * Alfred P. Sloan Research Fellow. been created as a result of previous dissociation. In the case of CF3I, this reaction can be virtually eliminated by performing the dissociation at temperatures low enough so that any 12 condenses at the cell walls [1]. Reaction (4), on the other hand, is likely to occur for all of the halides. In a previous publication [1] we have suggested that the recombination of CF 3 or I frag- ments via (4) might proceed more rapidly than reaction (1) under the conditions typically employed for laser isotope separation. The rate constant for reaction (1) has been reported [7-11] to be in the range (0.5- 3.8) x 10 - *1 cm 3 molec- * s - 1, while that for reaction (4) (X = I) has been reported [11-14] to be in the range (1.3-6.5) x 10-11 cm 3 molec - 1 s - 1. If reaction (4) does, in fact, compete with reaction (1), then the efficiency of the isotope separation process can be improved by finding a scavenger which will remove the CF 3 radicals more rapidly than they recombine with the free ha- logen atoms. In this paper we have reinvestigated the separation of carbon isotopes by multiphoton dissociation of CFaI. By using HI as a scavenger for the CF3 radicals, we have shown that the efficiency of the separation can be improved by at least 33 %. It is found that reactions (1) and (4) have approximately equal rate constants when X=I and that these rate constants are a factor of 8 higher than that for CF 3 +HI--+CF3H + I. 0340-3793/81/0024/0143/$01.00