Photodissociation of uracil

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 pi pi* 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 pp* state most likely deactivates via the lower-lying n pi* state. In addition there is evidence for a second pathway: direct decay of the pp* state to the electronic ground state with subsequent dissociation. Experiments on uracil-1,3-D-2 show that there is no site selectivity in the dissociation process. No evidence was found for the direct dissociation via a pi sigma* 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.