Iodine recombination cage effects

Otto B, Schroeder J and Tree J 1984 Photolytic cage effect and atom recombination of iodine in compressed gases and liquids experiments and simple models J. Chem. Phys. 81 202... 

There have been several studies of the iodine-atom recombination reaction which have used numerical techniques, normally based on the Langevin equation. Bunker and Jacobson [534] made a Monte Carlo trajectory study to two iodine atoms in a cubical box of dimension 1.6 nm containing 26 carbon tetrachloride molecules (approximated as spheres). The iodine atom and carbon tetrachloride molecules interact with a Lennard—Jones potential and the iodine atoms can recombine on a Morse potential energy surface. The trajectives were followed for several picoseconds. When the atoms were formed about 0.5—0.7 nm apart initially, they took only a few picoseconds to migrate together and react. They noted that the motion of both iodine atoms never had time to develop a characteristic diffusive form before reaction occurred. The dominance of the cage effect over such short times was considerable. 

Flash photolysis and electron-pulse techniques may be considered as cases of extreme perturbing functions, in the first case an extremely intense flash of light, in the other case a beam of electrons. Such perturbations cause extreme deviations from equilibrium concentrations, so that linear first-order rate equations no longer describe the time behavior of the system. In fact, molecules are often promoted to higher electronic states. With the advent of the laser, the time resolution of flash photolysis has been reduced to picoseconds. This permits the study of processes not previously accessible to kineticists. The types of systems studied include radiationless transitions, the solvated electron and chlorophyll. For example, the cage effect in liquids has been demonstrated by a study of the recombination of iodine atoms at very short times [8]. Although these methods are of considerable interest, we do not discuss them in further detail here (cf. Hammes [1]). 

Iodine.—Kinetic data on the reactions of iodine atoms with iodide ions and with nitrite ions have been obtained from flash-photolytic studies, and there has been a picosecond study of the dynamics of the cage effect in the recombination of iodine atoms. Iodine has recently been used as a cyanide-ion scavenger to investigate the loss of cyanide from inert transition-metal complexes, and so it is particularly appropriate that the kinetics of the reaction between cyanide and the tri-iodide ion should have been published [equation (29)]. The first term simply represents a... 

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