Effect of plasma screening on electron impact excitation and ionization of Fe16+ in a dense environment

Electron impact excitation and ionization with atoms and ions within a dense plasma are fundamental microscopic processes that determine the ionization balance, physical properties (such as electron conductive opacity and thermal conductivity) and plasma formation and dynamics. While collision cross-sections and rates are well studied in dilute systems, similar investigations are scarce for dense plasmas under stellar interior conditions using an appropriate plasma-screening potential. Here we investigate the plasma-screening effect on the electron impact excitation and ionization cross-sections, effective collision strengths, and rate coefficients within plasmas under stellar interior conditions in a mass density range of 1-15.748 g cm(-3) and a temperature range of 200-1000 eV. These investigations were carried out using our recently developed plasma-screening model, taking Fe16+ as an example. The results show that the cross-sections of the electron impact excitation are generally decreased, whereas they are always significantly increased for the collision ionization due to the plasma screening. In a plasma at a temperature of 200 eV and density of 15.748 g cm(-3), the plasma screening causes a decrease in the excitation cross-section of 36 per cent for the dipole-allowed transition 2s(2)2p(6) S-1(0) -> 2s(2)2p(5)3d(1)P(1)(0) and of 50 per cent for the dipole-forbidden transition 2s(2)2p(6) S-1(0) -> 2s(2)2p(5)3d(3)D(1)(0). However, the collision ionization cross-section of a 2p electron from the ground level of Fe16+ is increased by 500 per cent and 100 per cent under an incident electron energy of 1500 and 10 000 eV, respectively. This results in the rate coefficient increasing by a factor of 18.5 at a temperature of 200 eV and density of 15.748 g cm(-3).

Automated summary

This study investigates the impact of plasma-screening on electron impact excitation and ionization cross-sections, effective collision strengths, and rate coefficients in dense plasmas under stellar interior conditions. The results show that plasma screening leads to a decrease in electron impact excitation cross-sections while significantly increasing collision ionization cross-sections.