Absolute effective cross sections of ionization of adenine and guanine molecules by electron impact

Effective cross sections of the formation of positive ions of nitrous nucleic acids of adenine and guanine are determined by the crossed electron and molecular beam method in the energy interval from the threshold to 200 eV. It is found that the maximal value of the total cross section of adenine ionization is attained at an energy of 90 eV and is equal to (2.8 $\pm$ 0.6) $\times$ 10$^{-15}$ cm$^2$. The maximal value of the total cross section of guanine ionization is equal to (3.2 $\pm$ 0.7) $\times$ 10$^{-15}$ cm$^2$ and is observed at an energy of 88 eV. The energy ionization thresholds are determined, which amount to (8.8 $\pm$ 0.2) eV for adenine and to (8.3 $\pm$ 0.2) eV for guanine. The adenine and guanine mass spectra are measured. The absolute values of partial ionization cross sections of adenine and guanine molecules are determined.