Electron Impact ionization of the uracil molecule

With the recently rising interest in the properties of bio and astro molecules, a series of measurements concerning inelastic electron scattering (ionization / attachment) on these molecules were started. First measurements for the uracil molecule C4H4N2O2 were carried out. Besides electron attachment experiments at electron energies from 0 to 15 eV, the investigation of positive ion formation and dissociation at higher electron energies in a range from 8 to 30 eV was performed. A hemispherical electron monochromator with a maximum energy resolution of about 30 meV and a maximum electron energy of 600 eV is the main part of the used apparatus. The interaction between the uracil and the monochromatized electron beam occurs in the collision chamber, after the crossing point, the ions produced are mass separated by a quadrupole mass spectrometer. The rather low intensity of the evaporated uracil in the collision chamber was the major problem encountered. It was necessary to move the nozzle of the evaporation oven as near as possible to the crossing zone with the electron beam. With this experimental setup was possible to determine ionization appearance energies for atoms and molecules up to to charges states of about 7. The electron impact ionization cross section was measured as a function of the electron energy from few eV below the threshold to about 3 eV above its onset. The measured raw data for the ionization cross section near the threshold were fitted with a nonlinear least squares fit procedure, which uses the Marquart-Levenberg algorithm. (nevyjel)