Dissociative ionization of the nucleosides thymidine and uridine by electron impact

Electron impact ionization of gas phase thymidine and uridine near the threshold is studied using a high resolution hemispherical electron monochromator. For the parent cations of thymidine and uridine, appearance energy values are obtained of 8.8 and 9.02 eV, respectively. Above a critical temperature the mass spectra recorded at the electron energy of 70 eV show differences depending on the temperature of the molecules prior to the electron interaction. Here, we present that appearance energies measured with high electron energy resolution are a sensitive tool to probe thermal decomposition of fragile molecules.