ON THE EVIDENCE FROM HIGH‐INTENSITY TWO‐PHOTON LASER PHOTOLYSIS FOR ENERGY TRANSFER FROM HIGH‐LYING QUASI‐RYDBERG STATES OF THYMINE TO ISO‐ENERGETIC STATES OF WATER

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Abstract
High intensity two‐photon picosecond excitation at 266 nm (effective energy 9.2 eV) has been shown to cause efficient photolysis of thymine in aqueous solution. Experimental results for the quantum yield of solvated electrons, θ (e−aq), and the net consumption of thymine, θ(‐Thy) have been interpreted (Oraevsky and Nikogosyan, 1985) as indicating a stoichiometric excess of solvated electrons over thymine radical‐cations. To account for this under conditions in which direct two‐photon photolysis of water is negligible, it has been concluded that it is necessary to postulate the occurrence of extensive energy transfer from high‐lying thymine states to dissociating/ionizing states of water. This Note demonstrates that: (i) the supposed excess of e−aqdepends on an implicit assumption concerning the secondary radical reactions (dimerization) such that ‐ΔTh+=ΔThy+; if secondary radicals dismutate then ‐Thy =½δThy+, leading a stoichiometric deficit of e−aq; and (ii) the experimental results can be accounted for by a mixed dimerization/dismutation mechanism, without invoking energy transfer. Thus there is no evidence unambiguously requiring the postulate of energy transfer. But if it occurs, stoichiometry places restrictions on the extent of transfer and the nature of the secondary radical reactions.