Averaged Collision Strengths Research Articles

A Review of Electron Impact Collisional Excitation Data for the Oxygen-Like Isoelectronic Sequence

Published data on electron impact cross sections for ions of the oxygen-like isoelectronic sequence from 1953 to the present are reviewed. All transitions for which tabulated data are available are listed and classified according to source, type, method, and year of publication. Where possible, Maxwellian averaged collision strengths are computed at a standard set of atomic charge scaled electron temperatures. Compatible effective collision strengths are assembled into data sets that allow interpolation in charge and electron temperature and are suitable for excited level population studies.

Collision strengths for dipole-allowed transitions in S II

Calculations of collision strengths for electron-impact excitations of S II from the ground state 3p3 4S0 to excited states 3p4 4P, 3d 4F, 3d 4D, 4s 4P, and 3d 4P were carried out using the R-matrix code described by Berrington et al. (1978) and the NIEM code described by Henry et al. (1981). Results are presented for the thermally averaged collision strengths for the five-state and six-state calculations. Convergence behaviors were examined by comparison with the six-state calculations and the previously obtained two-state calculations. Uncertainties for these transitions were estimated to be within 20 percent, except for the 4S0 - 3p4 4P transition in which a 40 percent uncertainty was estimated.

Thermally averaged collision strengths for Sr+

We use the Gauss-Laguerre method to perform Maxwell averaging of distorted-wave collision strengths for transitions among the lowest three terms of singly ionized strontium. Rate coefficients are readily derived from the thermally averaged collision strengths \ensuremath{\Upsilon}. By suitably scaling \ensuremath{\Upsilon}, we form reduced collision strengths ${\ensuremath{\Upsilon}}_{\mathrm{red}}$ that are finite at all temperatures. Each ${\ensuremath{\Upsilon}}_{\mathrm{red}}$ is plotted as a function of reduced temperature ${T}_{\mathrm{red}}$, which maps the entire range of T onto the interval [0,1]. We use the least-squares method to fit ${\ensuremath{\Upsilon}}_{\mathrm{red}}$ by a five-point cubic spline. This fitting procedure has an rms error of less than 1% and is performed on a microcomputer by an interactive program with graphic display.

A New AID for Interpolating and Assessing Collision Strengths and Rate Coefficients

AbstractA new way of critically assessing and compacting data for electron impact excitation of positive ions is proposed. This method allows one (i) to detect possible printing and computational errors in the published tables, (ii) to interpolate and extrapolate the existing data as a function of energy or temperature, and (iii) to simplify considerably the storage and transfer of data without significant loss of information. Theoretical or experimental collision strengths Ω(E) are scaled and then plotted as functions of the colliding electron energy, the entire range of which is conveniently mapped onto the interval (0,1). For a given transition the scaled Ω can be accurately represented - usually to within a fraction of a percent - by a 5 point least squares spline. Further details are given in (2). Similar techniques enable thermally averaged collision strengths upsilon (T) to be obtained at arbitrary temperatures in the interval 0 < T < ∞. Application of the method is possible by means of an interactive program with graphical display (2). To illustrate this practical procedure we use the program to treat Ω for the optically allowed transition 2s → 2p in ArXVI.

Improved excitation rate coefficients for the N = 2 and N = 3 levels of CA XIX and Fe XXV including fine structure

Reently calculated collision strengths, including relativistic and resonance effects, are employed to compute Maxwellian averaged collision strengths for 78 transitions involving states of principal quantum numbers 2-1 and 3-1 in Ca XIX and Fe XXV. These rate parameters are tabulated at temperatures of interest in astrophysical and labortory plasmas with radiation in the hard X-ray wavelength range. For some transitions, significant differences are found with the earlier calculations of Pradhan, Norcross, and Hummer (1981).