Relativistic Distorted Wave Approach to Electron Impact Excitation of Heavy Rare Gases Using a Complex Potential
Marucha, Alex Magembe
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Data on excitation of rare gases is important in the study of plasma displays, lighting and lasers. From literature, both relativistic and non-relativistic computations performed on electron impact excitation of low-lying states of rare gases often fail to give satisfactory agreement with available experimental data mostly at low impact energies and at intermediate scattering angles. With this in view, in the present study, we have applied relativistic effects in a fully-relativistic distorted-wave approach to excitation of the lowest lying resonance states of argon, krypton and xenon gases, by modifying the electron-atom interaction distortion potential in such a way that the complex part, the absorption potential, and the real part, which includes an energy dependent polarization potential, exchange and electrostatic potentials, form the complex distortion potential used in calculating radial wavefunctions. The atomic wavefunctions are constructed in the multi-configuration Dirac-Fock approach by modifying the general-purpose relativistic atomic structure code GRASP for numerical procedures. In this study, the WKB approximation is used to compute the free continuum electron wavefunctions which are then used in computing scattering cross sections and angular parameters using our program RDWBA1. Present results from this study predict that use of a complex distortion potential in the relativistic approach to excitation of argon, krypton and xenon generally lowers integral cross sections as impact energies of the incident electron increases, compared to those obtained using real distortion potentials only. For argon, the effect of the absorption potential, which accounts for loss of flux into other open scattering channels is more visible at electron impact energies above 50 eV, while for krypton, absorption becomes more dominant above 100 eV. For xenon, which is the heaviest of the three, absorption in the distortion potential generally has minimal effect on cross sections at impact energies below 50 eV then significantly improves these results when compared with experiments as kinetic energy of the electron increases. Furthermore, for all the rare gases under investigation, it is the energy dependent polarization potential adopted, that plays a major role in improving shapes of cross-sections at low and near threshold impact energies, where available distorted-wave methods fail to give satisfactory results when compared to experiments. We have also obtained angular correlation parameters lambda to predict the magnetic sublevel responsible for most excitations, and Stokes parameters to predict the polarization of the emitted photon during atomic decay. Cross section results obtained from this study are in good agreement with experiments at all impact energies under investigation, therefore it will be interesting to see how these cross sections vary when this present approach is used to investigate excitation of the metastable states of rare gases with both electron and positron impact.