Collision Strengths and Line Strengths for Transitions from the ls²3ℓ, Levels to the ls2ℓ'3ℓ" Levels in Li-Like Ions

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It is well known that radiation from the levels of the ls2ℓ2ℓ' and ls2ℓ3ℓ' configurations of Li-like ions form satellites of spectral lines from He-like ions that are of high interest for plasma diagnostic purposes. Recently Sampson et al (1984) have made calculations of the collision strengths for innershell excitation to these upper levels from the ls²2s and ls²2p levels for a large number of Li-like ions using a Coulomb-Born-exchange method. Here we consider the alternative mechanism for populating the ls2ℓ3ℓ' levels by collisional excitation from the ls²3s, ls²3p and ls²3d levels. Although the poDulations of the ls²3ℓ levels are lower than those of the ls²2ℓ levels, this is at least partially compensated for by the fact that the collision strengths for the ls²3ℓ − ls2ℓ'3ℓ" transitions are considerably larger on the average than those for the ls²2ℓ − ls2ℓ'3ℓ" transitions. The method of calculation is the same as that used by Sampson et al (1984), where all possible configuration mixing, parentage mixing and intermediate coupling type mixing among the states in a complex (the states having the same n values, parity and J values) was included. Also the present application is sufficiently simple that after summation over J values for the initial level the results for the collision strength Ω can be expressed in a simple manner in terms of the Z scaled hydrogenic collision strengths Z²Ω_H and analogous to Eqs (8)-(15) of Sampson et al (1984). The appropriate formulae and the numerical results for both excitation to upper fine structure levels and to upper energy terms for 19 Li-like ions have been submitted for publication to Atomic Data and Nuclear Data Tables. Here we give only the pertinent formula and sample numerical results for transitions between energy terms in Li-like iron. This formula is

where due to configuration mixing neither ℓ need equal ℓ" nor ℓ_a need equal ℓ'. The recommended values for σ and σ^e are zero for transitions involving ls−2s and are −0.5 and 0.3, respectively, for transitions involving ls−2p. Numerical values for the coefficients A and A^e for Li-like iron are given in Tables I-III. Values for Z²Ω_H and for impact electron energies up to 15 times threshold are given in Golden et al (1981) and Clark et al (1982) and are repeated in Table IV for convenience. Finally we note that the line strengths S for transitions involving ls−2p are obtained by multiplying the corresponding value for A(ls−2p) by the scaled hydrogenic line strength 1.665 and dividing by (Z-σ)².