Ionization and dissociative ionization of adenine molecules by electron impact near threshold

Full text: An increased interest to the studies of biomolecules by traditional methods of physics of electron collisions is explained by the significance of these molecules in modern life. Here we report on a study of the features of the process of ionization, including dissociative ionization, of adenine molecules under electron impact, accompanied by the formation of ionized products of reaction. The experimental setup, used for the investigation of partial cross-sections of dissociative ionization of molecules by electron impact, is described in detail in a number of our papers. The setup is constructed on the base of a monopole mass spectrometer with an electron ionizer and a multichannel molecule source of effusion type. The ionic products of dissociative ionization, separated by the high-frequency field of the analyzer, were detected by a channeltron. The scanning of the ionizing electron energy and data acquisition were performed using a computer and a specially developed software. The duration of one measurement cycle was chosen in such a way that the number of pulses of the useful signal in the maximum of the energy dependence curve should not be less than 104. In short, the measurement technique was the following. At first the adenine molecule mass spectrum was measured at the ionizing electron energy of 40 and 70 eV , then for each fragment the dissociative ionization function was measured. The mass scale was calibrated using Ar, Kr and Xe, and a special procedure of polynomial fitting of the threshold part of the ionization cross-section was used to determine the appearance potentials of various ion fragment groups. The fragment appearance potentials were determined from the threshold dependences of the ion yield. We also studied temperature dependences of intensities of the ion fragments of the initial molecule in the temperature range 343-480 K. The measurement technique was reduced to the measurement of mass spectra at various temperatures at Eion = 50 eV. From the temperature dependences, the evolution of the fragment formation could be traced and the effect of temperature on the dissociative ionization could be observed. We have measured energy dependences of the total cross-section of the adenine molecule ionization as well as the cross-sections of dissociative ionization of formation of the fragment ions. We have also determined the appearance potentials for the fragment ions with m/e = 43, 54, 81, 108. (author)