Theoretical Study On The Electron-impact Excitation And Polarization Of Emitted X-rays For He-like Ions

The electron-impact excitation exists in astrophysics,thermonuclear fusion and various laboratory plasmas that is one of the basic process of atomic physics.They have important effects on energy transport,level populations of the excited state and radiation spectrum in plasmas.It is of great significance for accurate simulation and diagnosis of plasma state and obtaining high resolution dynamic parameters such as electron-impact excitation cross section,rate coefficient and linear polarization.Therefore,it is a hot topic in atomic physics to study the electron-impact excitation and radiation in detail.In this thesis,the electron-impact excitation process of He-like isoelectronic sequence ions and the polarization properties radiation are studied systematically using a the relativistic configuration interaction（RCI）theory method.The main contents are given as follows:Firstly,we calculated the excitation energy and transition probability from the ground state to the low excitation configuration 1s2l（l（28）s,p）,and 1s3l（l（28）s,p,d）of fourteen He-like ions by using the Flexible Atomic Code（FAC）program based on the relativistic configuration interaction theory.By comparing with the NIST results,excellent agreements are obtained for the excitation energy and transition probability of He-like ions.Based on the transition probability data,the level width and branching ratio can be obtained.We have made comparisons between different types of transition,including1s2s ¹S₀（?）1s2 p³ P₁ allowing transition（E1）,1s2s ¹S₀（?）1s2s ³S₁ forbidden transition（M1）and1s2s ¹S₀（?）1s^{2 1}S₀ two-photon transition.It is found that1s2s ¹S₀（?）1s^{2 1}S₀ two-photon transition cannot be ignored,when calculating the level width of the excited state1s2s ¹S₀.Secondly,we calculated the electron-impact excitation cross sections from the ground state to the individual magnetic sublevels of the excited state 1s2l（l（28）s,p）and1s3l（l（28）s,p,d）of He-like ions at electron-impact energy near 1.23 times the thresholds for the 1s^{2 1}S₀-1s2s ³S₁ transitions.The present results are in good agreement with the previous theoretical results.The total and magnetic sublevel cross sections of He-like isoelectronic sequence ions as functions of the atomic number Z are studied systematically.Furthermore,the cascade contributions from the high-lying levels1snl（n（28）2,3）to the cross sections were investigated.The results show that the cascade effect have much more outstanding contribution to the electron-impact excitation cross sections from the ground state to the individual magnetic sublevels of the excited state1s2s ³S₁,and bigger than the direct electron-impact excitation magnetic sublevels cross section.Finally,based on the work discussed above,the linear polarization of He-like ions was calculated.The cascade contributions from the high-lying levels 1snl（n（28）2,3）to the linear polarization of w line（1s2 p ¹P₁（?）1s^{2 1}S₀）,x line（1s2 p ³P₂（?）1s^{2 1}S₀）,y line（1s2 p ³P₁（?）1s^{2 1}S₀）and z line（1s 2s ³S₁（?）1s^{2 1}S₀）were investigated.It was found that the radiative cascades dominate the degree of linear polarization of the z line.The y line is strongly dependent on the atomic number at low-Z.