The Dynamic Screened Effect For Positron- Impact Ionization Of Atoms And Molecules

The interaction of positron colliding with atom or molecule is a basic aspect of the field of atomic physics, and positron plays an important role in material science, information science and space science field. The study of the interaction of positron with atoms have been studied many years. There were many experiments have been measured by some labrotaries, the single and double ionization cross section have been carried out for different experiments. In theory, the calculation methods are mainly quantum mechanics and classical mechanics to calculate the single ionization cross sections for positron-atoms. Because the process of muti-ionization of positron-atoms collision are not clear, the calculation of double ionization cross section for positron colliding with atoms is less studied, except some study near the threshold and the calculation of the differential cross section of double ionization at one energy.Our work are based on classical over barrier ionization model, and considering the screening of target electron for nuclear is used to calculated the trajectory of positron when approching to one atom. The single ionization process seemed to be a process that positron is scattering from a screened nuclear. The multi-ionization of atom is sequenced process, and the ionized electron will lose its screen abability to the nuclear. Therefore, the ionization of the second electron can be considered to be the process that the screened positron is scattered by the rest ion. We used this method to calculate the single and double ionization cross section of positron-Helium collision and the calculation results are in good agreement with experimental data. We find that the strong Coulomb potential makes the positron-induced double ionization of Helium suppressed at low to intermediate velocities.In this work, the collision of positron with atoms and ions are first calculated, including the single ionization of positron with H, He, Ne and He+ ion, and the double ionization of Helium. The calculations are in good agreement with the experimental data, which indicates the COBI and trajectory modification can use to calculate the single ionization process of positron-atom collision. In order to study the similarity and difference of positron and proton-atom collision, the scattering probabilities of positron and proton are calculated. We find that the positron feel a much strong Coulomb repulsion and the cross section smaller than proton at low velocities. At higher velocity, positron’s trajectory can be approximated to be linear, which is same to proton, so the cross sections are merging with each other. By comparing the scattering probabilities of positron-Helium single and double ionization, we find the scattering angles in double ionization at low velocities are very large. It indicates that the double ionization of positron-induced of Helium are strongly suppressed at low velocities. Finally, the single ionization of positron to hydrogen molecule is calculated and the results are in good agreement with experimental data.