Geminate recombination of molecular iodine. The role of A and A′ states

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Abstract
Picosecond absorption spectroscopy has been used to study the geminate recombination of I2 in room temperature solution. In all solvents studied (CCl4, CH2Cl2, CHCl3, hydrocarbons, and ethylene glycol) a transient absorption band in the 575–800 nm region was observed, and assigned to transitions originating from A(3Π1) and/or A′(3Π2) states. No spectral shift of this band was observed throughout its lifetime, making an assignment to a vibrationally unrelaxed ground state unlikely. Branching ratios for ground state vs A and A′ state recombinations were found to depend upon solvent viscosity. These results are explained in terms of the internuclear separation obtained upon dissociation. The geminate recombination rates into A or A′ states were found to be >(30 ps)−1 and ∼(90–140 ps)−1 into the ground state. At present we can not determine whether surface hopping to the ground state or subsequent diffusion is the rate limiting step in direct ground state recombination. Intersystem crossing rates from the A, A′ states to the ground state were found to vary from ∼(100 ps)−1 in hydrocarbons to (2.7 ns)−1 in CCl4.

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