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Electron scattering cross sections of pyrimidine calculated with the complex optical potential model

14.12.2012

The scattering of electrons by pyrimidine, a heterocyclic aromatic organic compound, has been increasingly studied recently, both experimentally and theoretically. As a basic component of the nucleobases cytosine and thymine, its electron scattering cross section is not only of importance for the determination of radiation damage in the DNA, but it is also suited to check the reliability of theoretical models - due to its relatively high molecular polarizability, to the high permanent dipole moment and the large spatial dimension of the charge distribution [1].

The total electron scattering cross sections of pyrimidine have been calculated with the aid of the complex optical potential model [2] for electron energies between 5 eV and 1 keV. The charge distribution computed by means of the quantum-chemical program Gaussian09 [3] and the single-center expansion method, taking the symmetry of the molecule into account, were used to obtain the optical potential. Then, the corresponding Schroedinger’s equation was solved by applying the variable-phase method [4].

Since pyrimidine has a relatively high permanent dipole moment, electric fields of the incident electron can induce the molecule to a rotation. In order to determine the electron scattering cross section due to the rotational excitation, the rotational energy levels of pyrimidine were at first calculated, and then the scattering cross section was computed with the aid of the Born approximation.

The total electron scattering cross sections were determined by summing up the cross sections for elastic scattering, for inelastic scattering with electronic excitation, and for inelastic scattering with rotational excitation. The figure shows the results in comparison to the experimental data. It can be seen from the figure that the relative energy dependence of the experimental results is well reproduced by that of the theoretical values.

Figure : Relative energy dependence of the experimental () total electron scattering cross sections of pyrimidine. The continuous line represents the energy dependence of the theoretical values, calculated with the aid of the complex optical potential model. These two data sets are normalized to the value at 100 eV.

Literature

  1. P. Palihawadana, J. Sullivan, M. Brunger, C. Winstead, V. McKoy, G. Garcia, F. Blanko, and S. Buckman, Phys. Rev. A 84, 062702 (2011)
  2. A. Jain and K. L. Baluja, Phys. Rev. A 45, 202 (1992)
  3. Gaussian 09, Gaussian, Inc., Wallingford, USA
  4. F. Calegro, Variable Phase Approach to Potential Scattering (Academic Press, Newyork, (1967)