Further calculations on electron excitation of forbidden lines: transitions with electric quadrupole moments

Abstract

Collisional excitation of the ground configuration terms of various ions such as O I, O II, O III, NI, N II and S II is of importance in various cosmic sources. The collision strengths defined as Ω,( n ', n ) = k 2 n w n Q ( n → n ′)/ π , have been calculated previously for a number of transitions dependent on pure exchange coupling. The present paper is concerned with the potential coupling terms for those transitions having electric quadrupole moments. For partial waves other than the p-wave the coupling strength is sufficiently weak for the distorted wave (D.W.) approximation to be used. The general theory of the D.W. approximation is discussed for complex atoms. It is shown that the angular and spin integrations occurring in the expressions for the collision strengths may be expressed in terms of the angular and spin integrations occurring in the calculation of electric multipole moments. Results of detailed numerical calculations are considered to be correct to within 30 to 40 %.