Electron localization induced increase in the electron impact excitation cross sections and rate coefficients of ions embedded in a dense plasma

The electron impact excitation process is important to investigate the ionization balance, the dynamical evolution of non-equilibrium plasmas, and the physical properties of plasmas. In dense plasmas, previous theories show that the electron impact excitation cross section is generally decreased compared with that in the case of isolated atoms or ions which applies to a dilute system. Investigations show that such decrease is caused by plasma screening. In this study, we propose a mechanism which increases the electron impact excitation cross section in dense plasmas. Due to the random collisions of the scattering electrons with other free electrons and ions in the plasma, the wavefunction of the scattering electrons can no longer be described by employing a plane wave distorted only by the scattering potential. The momentum of the scattering electrons is no longer a constant but changes with their distribution in a certain range, resulting in a phenomenon of transient spatial localization. The momentum broadening of the scattering electron is proposed to quantitatively describe such a localization. A theoretical formalism is developed to consider the transient spatial localization effect in the atomic collision theory and is applied to investigate the electron impact excitation processes of 1S(2) S-1(0) -> 1s2p P-1(1)0 and 1s(2) S-1(0) -> 1s3p P-1(1)0 of Si12+ embedded in dense plasmas. The results show that the calculated cross sections and rate coefficients are decreased by plasma screening, whereas their quantities are significantly increased in the dense plasmas by the transient spatial localization compared with those obtained by the isolated-atom model. These research findings provide new insight into the microscopic atomic process of the electron impact excitation and macroscopic physical properties such as the electron conductive opacity and the thermal conductivity.