The Primary Quantum Yield of Dissociation of Iodine in Hexane Solution

Abstract

The primary quantum yield for the photo-dissociation of iodine in degassed hexane at 25° has been calculated from measurements of the mean lifetime of the chains involved in the exchange of iodine atoms with trans-diiodoethylene. The primary quantum yield to an accuracy of ±15 percent is 0.59 at 436 mμ and 0.37 at 578 mμ. The deviation of this quantity from unity is affected both by the ``primary recombination'' of atoms which fail to escape from the original ``cage'' of solvent molecules, and by the ``secondary recombination'' of the original atoms by diffusion during a time which is very short compared to that in which either is apt to encounter an atom from another molecule. The specific rate constant for the recombination of iodine atoms in hexane solutions has been determined and has been shown to be about one fifth of the rate constant for collision in the gas phase. The presence of air decreases the rate constant for the photochemical exchange of iodine with diodoethylene by a factor of four and increases the apparent mean lifetime of iodine atoms by a factor of one hundred.