Secondary electron emission is the most important stage in the mechanism of radiation damage to DNA biopolymers induced by primary ionizing radiation. These secondary electrons ejected by the primary electron impacts can produce further ionizations, initiating an avalanche effect, leading to genome damage through the energy transfer from the primary objects to sensitive biomolecular targets, such as nitrogenous bases, saccharides, and other DNA and peptide components. In this work, the formation of positive and negative ions of purine bases of nucleic acids (adenine and guanine molecules) under the impact of slow electrons (from 0.1 till 200 eV) is studied by the crossed electron and molecular beams technique. The method used makes it possible to measure the molecular beam intensity and determine the total cross-sections for the formation of positive and negative ions of the studied molecules, their energy dependences, and absolute values. It is found that the maximum cross section for formation of the adenine and guanine positive ions is reached at about 90 eV energy of the electron beam and their absolute values are equal to 2.8 × 10(-15) and 3.2 × 10(-15) cm(2), respectively. The total cross section for formation of the negative ions is 6.1 × 10(-18) and 7.6 × 10(-18) cm(2) at the energy of 1.1 eV for adenine and guanine, respectively. The absolute cross-section values for the molecular ions are measured and the cross-sections of dissociative ionization are determined. Quantum chemical calculations are performed for the studied molecules, ions and fragments for interpretation of the crossed beams experiments.

Production of positive and negative ions of cytosine molecules (nucleic acid base) has been studied using a crossed electron and molecular beam technique. The method developed by the authors enabled the molecular beam intensity to be measured and the electron dependences and the absolute values of the total cross sections of production of both positive and negative cytosine ions to be determined. It has been shown that the total positive cytosine ion production cross section reaches its maximal value of 7.8 × 10−16 cm2 at the 78 eV electron energy. Dissociative ionization cross sections have also been determined. The maximum total negative cytosine ion production cross section was measured to be 4.2 × 10−18 cm2 at 1.5 eV.

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Ionization and excitation of the uracil molecules by electron impact is investigated. Production of positive ions of uracil molecules (nucleic acid base) was studied using a crossed electron and molecular beam technique. The method developed by the authors enabled the molecular beam intensity to be measured and the electron dependences and the absolute values of the total cross sections of production of both positive ions to be determined. It is shown that the total positive uracil ion production cross section reaches its maximal value of (1.0 ± 0.1) × 10(-15) cm(2) at the 95 eV electron energy. Dissociative ionization cross sections were also determined. The luminescence spectra of isolated uracil molecules in the wavelength range of 200-500 nm under the influence of slow electrons are obtained. In the spectrum, more than 20 spectral bands and lines at 100 eV electron energy are observed. It is shown that the uracil radiation spectrum is formed by the processes of molecules dissociative excitation, dissociative excitation with ionization, excitation of electronic levels of the initial molecule and molecular ion.

The ejected-electron spectra arising from the decay of the 4p nln l n l 5 ¢ ¢   autoionizing states in Sr atoms have been studied precisely at the incident-electron energies close to excitation and ionization thresholds of the 4p 6 subshell. The excitation behaviors for 58 lines observed between 12 and 21eV ejected-electron kinetic energy have been investigated. Also, the ab initio calculations of excitation energies, autoionization probabilities and electron-impact excitation cross sections of the states 4p nln l n l 5 ¢ ¢   (nl = 4d, 5s, 5p; n l ¢ ¢ = 4d, 5s, 5p; n l   = 5s, 6s, 7s, 8s, 9s, 5p, 6p, 5d, 6d, 7d, 8d, 4f, 5g) have been performed by employing the large-scale configuration-interaction method in the basis of the solutions of Dirac-Fock-Slater equations. The obtained experimental and theoretical data have been used for the accurate identification of the 60 lines in ejected-electron spectra and the 68 lines observed earlier in photoabsorption spectra. The excitation and decay processes for 105 classified states in the 4p 5 5s nl 2 , 4p 5 4d nl 2 and 4p 5 5snln l ¢ ¢ configurations have been considered in detail. In particular, most of the states lying below the ionization threshold of the 4p 6 subshell at 26.92eV possess up to four decay channels with formation of Sr + in 5s 1 2 , 4d 3 2,5 2 and 5p 1 2,3 2 states. Two-step autoionization and two-electron Auger transitions with formation of Sr 2+ in the 4p 6 S 1 0 ground state are the main decay paths for high-lying autoionizing states. The excitation threshold of the 4p 6 subshell in Sr has been established at 20.98±0.05eV.