Selective Photochemistry of Bromine Using a Ruby Laser

Abstract

Gas‐phase photochemical addition of bromine to olefin molecules has been studied by inducing the reaction with monochromatic light near 6940 Å from a pulsed, tunable ruby laser. All previous photochemical reactions of bromine were induced with light at wavelengths shorter than 6800 Å, and were found to proceed by means of free radical chains. The Br atoms initiating these chains are produced by direct dissociation of Br₂ molecules upon absorption of light in the continuum. The present investigation shows that free‐radical chains are responsible for the reaction at 6940 Å, also. However, direct dissociation at this wavelength is found to be negligible, and the Br₂ molecules are excited to individual bound levels 500 to 800 800 cm^− 1 below the dissociation energy. Kinetic and isotopic evidence shows that the additional energy is furnished by subsequent collisions, so that about 1% of the excited Br₂ molecules become dissociated and can initiate the reaction. The remaining excited Br₂ molecules relax by collisions to the ground state at a rate somewhat higher than the gas–kinetic‐collision rate.