Crossed molecular beam studies of chemiluminescent reactions: F2+I2, Br2, and ICl

The chemiluminescent bimolecular halogen–halogen reactions, F2+I2, Br2, and ICl, have been studied by the crossed molecular beam technique. Undispersed chemiluminescence was measured as a function of collision energy and, for I2+F2, as a function of the two beam pressures. Although no spectra were obtained to positively identify the emitters as IF*, ClF*, and BrF*, arguments are given to support this identification. The observed reaction thresholds of 4.2 and 5.9 kcal/mole for I2+F2 and ICl+F2, respectively, are the same as the threshold energies for production of the stable trihalogens I2F and ClIF. This coincidence of threshold energies, as well as similar high collision energy behavior, implies that the chemiluminescent reaction proceeds via a stable trihalogen intermediate. This mechanism can explain our results and the results of other workers without resorting to a symmetry forbidden four center reaction mechanism. A threshold of 11.3 kcal/mole was found for Br2+F2; no threshold for Br2F has been previously reported. Laser enhancement of the I2+F2 reaction was attemped, but no enhancement was seen.