Electron-impact excitation of complex atoms and ions

Abstract

A new R-matrix approach for calculating cross sections and rate coefficients for electron-impact excitation of complex atoms and ions is described. This approach, based on an expansion of the total wavefunction in target configurations rather than in individual target states and taking advantage of the special status of the scattered electron in the collisional wavefunction, enables the angular integrals to be performed very much more efficiently than hitherto. It also enables electron correlation effects in the target and in the electron-target collision complex to be treated consistently, eliminating pseudo-resonances which have caused serious difficulties in some earlier work. A major new program package RMATRX II has been written that implements this approach and, as an example, electron-impact excitation of Fe2+ is considered where the four target configurations 3d6, 3d54s, 3d54p and 3d54d are retained in the expansion of the total wavefunction. RMATRX II is compared with the standard R-matrix program package and is found to be much more efficient showing that accurate electron scattering calculations involving complex targets, such as the astrophysically important low ionization stages of iron-peak elements, are now possible.

Electron-impact excitation of Ni II

“…The RMATRX II R-matrix package (Burke et al 1994) was used to complete the internal region calculations. This suite of codes allows for a more complete treatment of correlation effects in the internal region than would be possible with other packages.…”

“…This suite of codes allows for a more complete treatment of correlation effects in the internal region than would be possible with other packages. RMATRX II (Burke et al 1994) utilises LS -coupling in the internal region. Working in LS at this stage ultimately results in more available computing memory and this in turn permits the inclusion of more sophisticated CI wavefunctions representing both the target and the (N + 1)-scattering system.…”

“…The subsequent work of Watts et al (1996) presented the first application of the RMATRX II R-matrix package (Burke et al 1994) to electron-impact excitation of a lowly ionized open d-shell system. An extensive LS collision study was carried out including 27 doublet and quartet levels arising from the 3d 9 , 3d 8 4s and 3d 8 4p Ni ii basis configurations, a much larger close coupling (CC) expansion than that adopted by Bautista & Pradhan (1996).…”

“…In the present work, we embark upon the largest theoretical study of fine-structure collisional data for Ni ii and report the first application of the recently extended RMATRX II R-matrix package (Burke et al 1994) in the internal region which now accounts for relativistic fine-structure effects via FINE (Burke, V. M., private communication). The PSTGF program (Ballance & Griffin 2004) is utilised in the external region.…”

Electron impact excitation of Mg VIII

“…M2 was developed to examine if having a larger CI expansion, and thus slightly better energies, provided a substantial improvement over M1. Internal region information for M2 was obtained in LS coupling, through use of the RMATRXII (Burke et al 1994) suite of codes (ANG, RAD and HAM). The code FINE (Burke, priv.…”