The electronic spectra of the pyrimidine bases uracil and thymine in a supersonic molecular beam

Abstract

The resonance-enhanced multiphoton ionization spectra of uracil and thymine have been observed in the gas phase in a supersonic molecular beam. In spite of extensive cooling the spectra are broad and diffuse. These results are compared to a previously reported fluorescence spectrum for uracil which showed sharp structure. Photoionization spectra show that the species responsible for the sharp spectrum has a higher molecular weight than uracil, and this species appears to be an impurity formed from uracil in the oven. The diffuseness in the uracil and thymine spectra is attributed either to mixing of electronic states or to a large geometry change between the ground and excited electronic states.