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Integral electron scattering cross-sections of cytosine and thymine

02.01.2012

In addition to the integral elastic electron scattering cross-sections of the nucleic bases cytosine and thymine, which have been determined by numerical integration of the differential elastic electron scattering cross-sections over the scattering angle calculated on the basis of the Modified Independent Atom model [1], their total electron scattering cross-sections and integral ionization cross-sections have been determined theoretically and semi-experimentally with an approximation model.

Measurement results obtained so far for the total electron scattering cross-sections of several organic molecules show that for electron energies above 20 eV, the ratio of the total electron scattering cross-sections to the number of valence electrons scarcely depends - within the scope of the measurement accuracy - on the type of the molecule provided that the valence electrons are delocalized to a high degree. This applies to molecules with a ring structure such as tetrahydrofurane, pyrimidine, cytosine and thymine. This feature was used to determine the total electron scattering cross-sections of cytosine and thymine from those of pyrimidine semi-experimentally.

The ionization cross-sections of cytosine and thymine were calculated with the aid of the Binary Encounter/Bethe model [2]. The respective data required for the mean kinetic energy and the binding energy of the electrons in different molecule orbitals were determined with the aid of the quantum chemical programme GAUSSIAN 09 [3]. The figure shows the total, integral elastic cross-sections and the ionization cross-sections of cytosine and thymine as a function of the electron energy T.

Figure : Total, integral elastic cross-sections and ionization cross-sections of cytosine (a) and thymine (b): () ionization cross-section, (•) integral elastic electron scattering cross-section, () total electron scattering cross-section, () sum of the integral elastic cross-section and of the ionization cross-section.

Literature

  1. S. Hayashi and K. Kuchitsu, J. Phys. Soc. Japan 41, 1724 (1976)
  2. Y.-K. Kim and M. E. Rudd, Phys. Rev. A 50, 3954 (1994)
  3. Gaussian 09, Gaussian Inc., Wallingford CT, USA, 2009