Comment on “Experimental and theoretical study of the triple-differential cross section for electron-impact ionization of thymine molecules”

Abstract

In their recent paper, Bellm et al. [Phys. Rev. A 85, 022710 (2012)] performed (e,2e) experiments on thymine at an incident energy of 250 eV. They wrote in the conclusion that a model based on the first Born approximation using the completely neglected differential overlap description is in very good agreement with the experimental data. On the contrary, we argue that this model fails to describe experiments on water performed at the same incident energy and is unable to explain any shift of the binary or recoil peaks.The ionization of molecules of biological importance is a fundamental step to understand the interaction of charged particles with the human body. The (e,2e) experiments, which detect the scattered and the ejected electrons in time coincidence, are an important tool to investigate the tripledifferential cross section (TDCS) which provides a stringent test of an ionization process.Bellm et al.[1] performed difficult (e,2e) experiments on inner valence orbitals of thymine at an incident energy of 250 eV and an ejected-electron energy of 20 eV. The difficulties are due to the measurements of inner valence orbitals of thymine which are very closely spaced in energy. Therefore, it is difficult to separate each molecular orbital of thymine. Presumably, their measurements contain contributions from eight molecular orbitals. Moreover, we notice that there are two conformers of the thymine molecule with small differences in orbital ionization between these two structures [2]. Thus one may assume that these differences are small for the molecular wave functions too. It is well worth noting that the experiment of Bellm et al.[1] suffers from one drawback: all the measured data are relative.In the section of theoretical framework Bellm et al.[1] write that in their work on the (e,2e) experiment with coplanar asymmetric kinematics, the Coulomb Born approximation, also referred to as the first Born approximation (FBA), is completely justified. It is not quite true, because we notice that at the scattering angle of 10 • (see Fig. 3 of Ref.[1]) the TDCS shows a single binary lobe centered close to the momentum transfer direction with a small shift of approximately 5 • to larger ejected-electron angles. As the TDCS provides the most detailed information of an ionization process, it is impossible for a simple method like FBA to reproduce all the features of an (e,2e) experiment. For example, Brauner et al.[3] introduced a model, usually referred to as the Brauner-Briggs-Klar (BBK) * Corresponding author: s_houamer@univ-setif.dz model, to calculate the TDCS for the ionization of atomic hydrogen at a high incident energy of 250 eV (which is nearly 18 times the ionization threshold) and for a fixed ejectedelectron energy of 5 eV in a coplanar geometry, and compared their results with the corresponding absolute measurements of Ehrhardt et al. [4] and FBA calculations. They found that FBA failed to reproduce the shifts in the binary and recoil peaks observed in the experiment. The reason is that FBA cross sections are sym...

Angular distributions in the double ionization of DNA bases by electron impact

“…We notice a relative good agreement with the relative experimental data of Bellm et al [4]. A complete study of the single ionization of thymine can be found in [86][87][88].…”

Triple differential cross sections for the ionization of tetrahydrofuran by electron impact

“…In the past recent years, extensive research has been conducted to measure and calculate the triple differential cross sections (TDCSs) for molecules of biological interest [6][7][8][9][10][11][12][13][14][15][16].…”

Ionization of phenol by single electron impact: triple differential cross sections

“…One of these methods is the multi-center distorted wave model where non-diagonal terms in the potential matrix are ignored to make the calculations possible [27][28][29][30]. Another approximation is to calculate the TDCS as a weighted sum of atomic TDCSs corresponding to the atoms of the molecule, in the completely neglected differential overlap description, which fails in reproducing the experimental TDCS for many molecules [7,31]. To our knowledge, the methodology that we propose is the only one that allows the calculation of the TDCSs for complex targets without having to make such drastic approximations.…”