Recombination of iodine atoms in dilute solutions of argon

Abstract

The reaction 2I + Ar→ I 2 + Ar was studied at 298, 323 and 423 K by flash photolysis. The overall rate constant, k obs , for this reaction is a linear function of [I 2 ]/[Ar], but below [I 2 ]/[Ar] ~ 10 -4 , the relation becomes non-linear and k obs falls below extrapolated values. The fall-off is explained in terms of a mechanism involving an IAr intermediate: I + Ar ⇌ IAr IAr + I → I 2 + Ar. The equilibrium separation in such an IAr complex is 0.55 nm and the binding energy is 6.3 kJ mol -1 (1.5 kcal mol -1 ). This mechanism predicts the onset of the fall-off in agreement with the available experimental data. Moreover, the temperature dependence of this onset and the temperature dependence of the recombination rate constant from 298 to 1500 K are also satisfactorily explained. In addition to the above, the new mechanism yields quantitative agreement between our new rate constants and those reported previously. These combined data for 298 K yield a value of 3.00 (± 0.16) x 10 9 l 2 mol -2 s -1 for the rate constant for the reaction 2I + Ar → I 2 + Ar, and one of 1.00(± 0.09) х 10 12 I 2 mol -2 s -1 for 2I + I 2 → 2I 2 . Several other mechanisms are also considered, including some previously suggested in the literature. It is shown that none of these explains satisfactorily all the experimental data. However, most of the available experimental data could be explained, if it were assumed that the recombination proceeds via an unobserved electronically excited I 2 , formed from two 2 P 3/2 atoms, with a potential well 21 to 29 kJ mol -1 deep (5 to 7 kcal mol -1 ).