Recombination of iodine atoms by flash photolysis over a wide temperature range. VII. Recombination between 206 and 300 °K

Abstract

Recombination of iodine atoms was studied by flash photolysis of HI at 206 and 300 °K. The photolysis produced both H and I atoms. However, under present experimental conditions, the reaction H+HI→H2+I was completed within the duration of the flash, thus eliminating reactive hydrogen atoms and yielding additional quantities of iodine atoms. Condensation of solid I2 on cold walls of the reaction vessel was found to be much slower than iodine recombination reaction. For these reasons I atom recombination could be studied without interferance of side reactions. At 206 °K the recombination rate constants in He, Ar, and Xe were found to be (in 10−9l2 mole−2⋅sec−1) 2.8, 7.9, and 15.8, respectively, which agree well with the rate constants obtained earlier between 300 and 1200 °K [J. K. K. Ip and G. Burns, J. Chem. Phys. 56, 3155 (1972)]. Recombination rate constants in H2 and D2, 7.8×109 and 7.7×109 l2 mole−2⋅sec−1, respectively, agree reasonably well with earlier experimental results obtained at elevated temperatures [H. W. Chang and G. Burns, J. Chem. Phys. 59, 1043 (1973)]. It was found that the recombination rate constants of iodine atoms in HI can be expressed by the following equation, valid between 206 and 300 °K: logkHIr= 10.47−3.5log(T/300). This last reaction appears to occur via the radical–molecule complex mechanism. The intermediate radical in this reaction may be a relatively stable hydrogen bi-iodide, in which case the activation energy of the reaction IHI+I→IH+I2 should not be negligible. Alternatively, the intermediate could be an unstable I–IH radical, in which iodine atom may interact with both atoms of HI. However, in this case weak chemical forces involving interaction between two halogen atoms would predominate. At 487 nm iodine absorption coefficients ratio ε (206 °K)/ε (300 °K) is equal to 1.15.