Progress has been made on the development of a low-temperature source for the generation of uranium atoms, and the use of laser-induced predissociation in Rb/sub 2/* for isotope separation. However, our main efforts have concentrated on understanding and improving the laser isotope separation of /sup 35/Cl and /sup 37/Cl by photochemical means. The gas-phase reaction ICl* with 1,2-dibromoethylene produces both cis-1-bromo-2-chloroethylene and cis-1,2-dichloroethylene stereospecifically. Selective excitation of I/sup 37/Cl with a tunable cw dye laser causes identical degrees of isotope enrichment in both products. By the use of an I/sup 35/Cl cell placed inside the cavity of the dye laser, the /sup 35/Cl:/sup 37/Cl ratio may be changed from 3:1 (natural abundance) to greater than 1:15. A competing pathway, cis-trans isomerization of starting material, also occurs and is the fastest reaction. The stereospecificity of the reaction and the relative rate of geometrical isomerization to substitution are the same when chlorine atoms, formed by the photodissociation of ICl or Cl/sub 2/, are scavenged by dibromoethylene. We conclude that, kinetically, ICl* acts as a free chlorine atom. That isotope separation is achieved by a free-radical scavenging mechanism is indeed remarkable and owes its success to the structure of the scavenger and themore » choice of initial conditions.« less