The collision strengths are completely dominated by resonances. Since the aim of this calculation is to provide collision rates the delineation of the resonances has to be detailed enough so as to avoid errors when the collision strengths are sampled by the Maxwell velocity distribution function. This is particularly critical near the excitation threshold where we used small steps with the effective quantum number relative to the nearest higher threshold. When resonances due to the corresponding threshold were averaged using the Gailitis method (Seaton 1983) and was taken relative to the next higher threshold. At energies between about 1 Ryd and 1.5 Ryd the closely-packed target states made this approach impractical and instead we used a small constant step in energy. In order to ensure that no essential information was lost the calculation was repeated at points shifted by half that steplength. This process was repeated with the steplength halved until it was found that the results obtained from two sets of non-overlapping data points differed by less than 1%. The final results were obtained by integrating over all datapoints.

Figure 2:
Low energy collision strength for excitation of
3s^{2}3p(^{2}P_{1/2} - ^{2}P_{3/2}) in P III, obtained using a 22
term CC expansion. DW calculations by
Krüger & Czyzak (1970) |

These scattering calculations were performed in LS coupling and in order to obtain collision strengths for the fine-structure transition the T-matrix elements were transformed algebraically to pair coupling. The spin-orbit interaction between the target terms was included as a perturbation by a second transformation that incorporated the so-called term coupling coefficients (Saraph 1978). In practice, at the low energies considered here the collision strengths are hardly affected by term coupling.

Figure 3:
Low energy collision strength for excitation of
3s^{2}3p(^{2}P_{1/2} - ^{2}P_{3/2}) in S IV, obtained using a 21
term CC expansion. DW calculations by
Krüger & Czyzak (1970) |

The fine-structure splitting of the target terms was neglected. This leads to some inaccuracy in the collision rate at the lowest temperature, but is not as serious for these light ions as for the ions discussed in IP XI due to the relatively smaller fine-structure splittings. The low energy collision strengths are shown in Figs. 2 to 4 where the results of the distorted wave calculations of Krüger & Czyzak (1970) are included for comparison.

Figure 4:
Low energy collision strength for excitation of
3s^{2}3p(^{2}P_{1/2} - ^{2}P_{3/2}) in Cl V, obtained using a 21
term CC expansion. DW calculations by
Krüger & Czyzak (1970), the point at 0.4 Ryd
was obtained by
interpolation |

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