Electron excitation collision strengths for transitions in K II

Abstract

Aims. Oscillator strengths and electron impact excitation collision strengths for transitions between the 43 fine-structure levels of the 3p 6 ,3 p 5 4s, 3p 5 3d, 3p 5 4p, 3p 5 4d and 3p 5 5s configurations in K II are calculated. Thermally averaged collision strengths are presented as a function of electron temperature for application to astrophysical plasmas. Methods. The B-spline Breit-Pauli R-matrix method is used to investigate the electron impact excitation of forbidden and allowed transitions in singly ionized potassium. The relativistic effects have been incorporated through mass, Darwin and spin-orbit one-body operators in the Breit-Pauli Hamiltonian in the scattering calculation, while in the calculation of oscillator strengths the one-body and two-body relativistic operators are included. Flexible non-orthogonal sets of spectroscopic and correlation radial functions are used to obtain accurate description of K II levels and to represent the scattering functions. The 43 fine-structure levels of the 3p 6 ,3 p 5 4s, 3p 5 3d, 3p 5 4p, 3p 5 4d and 3p 5 5s configurations have been considered in both the radiative and scattering calculations. Results. The present cross sections for the excitation of the lowest 4s and 3d states show reasonable agreement with the previous R-matrix calculations. The calculated excitation energies are in excellent agreement with experiment and represents an improvement over the previous calculation. The oscillator strengths for transitions from the ground state normally compare very well with previous calculations, but for transitions from the metastable state a large discrepancy was found with other calculations. The effective collision strengths are obtained by integrating total resonant and non-resonant collision strengths over a Maxwellian distribution of electron energies and these are presented over a wide temperature range.