Thymine relaxation after UV irradiation: the role of tautomerization and pi-sigma* states.

Ab initio calculations and femtosecond pump-probe ionization experiments were carried out to identify excited state relaxation processes in isolated thymine monomer and small thymine-water clusters. Three transient species with life times of < or =100 fs, 7 ps and >1 ns were observed in the experiments on gas phase thymine. The longer-lived transients were weak or absent in thymine-water clusters. Available theoretical results on thymine agree with the assignment of low-lying pi-pi* and n-pi* excited states to the femtosecond and picosecond transients but the assignment of the third transient remains opaque. Our theoretical results seem to exclude the possibility of ground or excited state tautomerization as well as the involvement of states with pi-sigma* character. Remaining explanations for the observed transients are: very fast intersystem crossing to the triplet manifold or the observation of transient signals from local minima on the potential energy surfaces.