Collision Strengths for Electron Scattering from Siii

Abstract

Electron impact excitation collision strengths and rates for transitions between fine-structure levels of the 3s 2 np (n ¼ 3 Y6), 3s3p 2 , 3s 2 ns (n ¼ 4Y 6), 3s 2 nd (n ¼ 3 Y5), 3s 2 nf (n ¼ 4 Y5), and 3s3p3d configurations in Si ii are calculated by using the Breit-Pauli B-spline R-matrix approach. The 31 target levels have been included in the closecoupling expansion in our collision calculation. The multiconfiguration Hartree-Fock method with term-dependent nonorthogonal orbitals is employed for an accurate representation of the target wave functions. The target levels have been described by using both spectroscopic and correlation radial functions. The atomic wave functions yield excitation energies that are in excellent agreement with experiment and other reliable calculations. The relativistic corrections are included through the one-body mass correction, Darwin, and spin-orbit operators in the Breit-Pauli Hamiltonian. The effective collision strengths have been calculated by integrating total resonant and nonresonant collision strengths over a Maxwellian distribution of electron energies, and these are presented over a wide temperature range suitable for modeling of astrophysical plasmas. Significant differences are noted with the previous eight-state R-matrix calculation, particularly for transitions involving higher excitation levels.

Radiative transition rates and collision strengths for Si II

“…The present results agree reasonably well with those of Dufton & Kingston (1991), who also used an orthogonal R-matrix method but with a much smaller close coupling expansion. On the other hand, the present results for Maxwellian averaged collision strengths are systematically lower than those of the recent calculation of Tayal (2008) using a non-orthogonal R-matrix method. The reasons for these differences, that typically amount to ∼ 30%, are unclear.…”

“…It is apparent that the present collision strengths should yield Maxwellian averages somewhat lower than those of Tayal. In Table 5 we compare the present fine structure Maxwellian averaged collision strengths with those of Tayal (2008) and Dufton & Kingston (1991). For most transitions our results are ∼ 30% lower than those of Tayal, while the results of Dufton & Kingston lie inbetween those two.…”

The Temperature and Ionization of Unshocked Ejecta in Cas A

“…Dielectronic recombination at very low densities will favor moderate l levels, in which case more of the cascades go through the 1 S 0 state, and a factor 0.21 would be more accurate, but we do not know which states contribute to the recombination. We use the collision strength for the [Si II] 35 µm line from Tayal (2008). The result is independent of the ionization fractions, because both lines are formed by collisions of electrons and Si + ions, and it is shown in Figure 4.…”

Chianti—an Atomic Database for Emission Lines. Xii. Version 7 of the Database

“…The CHIANTI model for Si ii has been updated to include all fine-structure levels from the 3s 2 3p, 3s3p 2 , 3s 2 3d, 3s 2 4l (l = s, p, d, f ), 3s 2 5l (l = s, p, d), and 3s 2 6l (l = s, p) configurations, together with the 3s3p( 3 P )3d 2 D J levels, yielding 29 fine-structure levels in all. Effective collision strengths are taken from Tayal (2008a), who provided data calculated at 14 temperatures between log T = 3.4 and 5.4. The assessment of the collision data performed for CHIANTI revealed problems for 13 transitions whereby the collision strengths showed discontinuous jumps at low temperatures.…”