Threshold electron impact ionization studies of uracil

Electron impact ionization near the threshold of the DNA base uracil (U) was investigated using a high resolution hemispherical electron monochromator combined with a quadrupole mass spectrometer. The mass spectrum measured at the electron energy of 70 eV reveals a rich fragmentation pattern. For the parent ion and some of the most intense fragment ions we measured the ion efficiency curves near the threshold and determined the corresponding appearance energies (AEs) using a nonlinear least square fitting routine based on a Wannier type power law. The present AE(U+/U)= 9.59 +/- 0.08 eV is in good agreement with vertical ionization energies obtained in previous photoelectron spectroscopy experiments. The appearance energies for the two most abundant fragment ions are AE(C3H3NO+/U)= 10.89 +/- 0.07 eV and AE(OCN+/U)= 13.41 +/- 0.10 eV. (C) 2004 Elsevier B.V. All rights reserved.