Photodissociation of uracil

Abstract

We investigate the photochemistry and photodissociation dynamics of uracil by two-colour photofragment Doppler spectroscopy and by two-colour slice imaging at excitation wavelengths between 268 and 235 nm. We observe the loss of a hydrogen atom upon excitation into the ππ* state. The angular distribution indicates a statistical process, while the translational energy distribution agrees with a dissociation that takes place on the electronic ground state. The ππ* state most likely deactivates via the lower-lying nπ* state. In addition there is evidence for a second pathway: direct decay of the ππ* state to the electronic ground state with subsequent dissociation. Experiments on uracil-1,3-D₂ show that there is no site selectivity in the dissociation process. No evidence was found for the direct dissociation via a πσ* excited state that seems to be relevant in the photochemistry of adenine and many other heterocyclic molecules. Overall, the photochemistry of uracil is similar to that of thymine.