Electron-impact excitation of complex atoms and ions

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 Fe 2+ is considered where the four target configurations 3d 6 , 3d 5 4s, 3d 5 4p and 3d 5 4d 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.