Ultrafast time-resolved spectroscopic studies of complex chemical reactions in solution are frequently hindered by difficulties in recovering accurate structural models for transient photochemical species. Time-resolved x-ray and electron diffraction have recently emerged as techniques for probing the structural dynamics of short lived photointermediates. Here we determine the structure of a transient isomer of photoexcited ${\mathrm{CH}}_2{\mathrm{I}}_2$ in solution and observe the downstream reactions of the initial photoproducts. Our results illustrate how geminate recombination proceeds via the formation of a transient covalent bond onto the iodine atom remaining with the parent molecule. Further intramolecular rearrangements are thus required for the ${\mathrm{CH}}_2\mathrm{I}\mathrm{\text{−}}\mathrm{I}$ isomer to return to ${\mathrm{CH}}_2{\mathrm{I}}_2$. The generation of $\mathrm{I}_3{}^{-}$ from those iodine radicals escaping the solvent cage is also followed with time.

• Received 11 November 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.245503