Studies of I Atom and Br Atom Recombination Following Flash Photolysis of Gaseous I2 and Br2

The method of flash photolysis has been used to determine the rate constant, k, in the expression d(X2)/dt = k(X)2M for the combination of iodine atoms in argon, and of bromine atoms in argon, oxygen, and nitrogen. At room temperature nitrogen is approximately 1.3 and oxygen 2.2 times as effective as is argon in serving as a ``third body'' for the combination of bromine atoms. Measurements of the rate constants at a series of temperatures up to 160° give apparent activation energies of —1.4 kcal/mole for I in A, —2.0 kcal/mole for Br in A and —1.2 kcal/mole for Br in O2. The (I2)/(A) ratios used in the iodine atom studies were in the range of the ``tail'' of the (I2)/(A) vs k plot of the Cambridge group and the k values are in agreement with their results. Estimates of the maximum ``thermal effect'' possible in the bromine atom studies, compared with the experimental results, indicate that such effects were probably negligible. Experimental tests and calculations of the effect of nonuniform illumination of the reaction cell by the flash lamp indicate that the observed rate constants are quite insensitive to this factor.