Femtosecond Photoelectron Spectroscopy of the Dynamics of Electron Attachment and Photodissociation in Iodide-Nucleobase Clusters

The attachment of low-energy secondary electrons to DNA is believed to be a major contributor to DNA damage. Theoretical calculations and dissociative electron attachment experiments predict that nucleobases are the most likely initial site of electron attachment, although the overall mechanism for attachment and subsequent bond breakage is not yet well understood. To probe the dynamics of this reductive DNA damage mechanism, we have employed femtosecond time-resolved photoelectron spectroscopy of gas-phase iodide-nucleobase and iodide-nucleobase-water clusters. A UV pump photon is used to initiate charge transfer from the iodide to the nucleobase moiety, and a UV or IR probe pulse is used to photodetach the excess electron from the photoexcited transient negative ions or the ionic photofragment dissociation products. In this way, the ultrafast photochemistry of excess electron accommodation in nucleobases and the subsequent photodissociation and relaxation pathways are monitored.