Photodissociation of thymine.

We discuss the photochemistry and photodissociation dynamics of thymine as revealed by two-colour photofragment Doppler spectroscopy and by one-colour slice imaging. Thymine is optically excited into the pipi* state, known to deactivate quickly. The H atom photofragment spectra are dominated by two-photon excitation processes with subsequent statistical dissociation. This can be explained by absorption of a second photon from a long-lived dark state to a highly excited state that quickly deactivates to the electronic ground state. No evidence was found for an important role of the pisigma* excited state identified in adenine and many other heterocyclic molecules.