Collisional Excitation Rate Coefficients for Lithium-Like Ions

Abstract

This report takes all the available good quality quantal calculations of excitation cross-sections by electron collision for lithium-like ions and intercompares them. There is a comparison also with the small amount of experimental data of 2s %-2p aP cross-sections. On the basis of all of these data, a choice is made of the best cross-sections and these are integrated over Maxwellians to give excitation rate coefficients. In general data are available for up to seven transitions in five or six ions. When the results are compared along the iso-electronic sequence, trends are established which allow estimates to be made of the rate coefficients for these seven transitions for any lithium-like ion of nuclear charge greater than boron. The results are presented graphically and as simple formulae. The formulae reproduce the source data at various levels of accuracy from about f 1% for individual ions to universal formulae of accuracy better than f 15% in the relevant temperature ranges. The latter are contained on one page and represent an enormous condensation of the original source data without seriously compromising the authors' claimed accuracy.

Massive stars at low metallicity

“…N  oscillator strengths for n < 6 were taken from Liang & Badnell (2011), the photoionization data were taken from Peach et al (1988), and collisional data for the 2s-2p, 2s-3 and 2p-3 transitions were taken from Cochrane & McWhirter (1983). N  oscillator strengths and photoionization crosssections were computed in Tully et al (1990) as part of the opacity project (Seaton 1987;Cunto et al 1993).…”

X-ray-heated models of stellar flare atmospheres - Theory and comparison with observations

“…Specifically, we can write the energy emissivity per unit volume for a resonance fine as R = n e n h X el <S> iOD (T)(t>Mí-'r X lO" 3 )/^-1 / 2 X exp ( -«/kT)e/e eV , 28where e and € eV are the energy of the transition in ergs and electron volts, respectively; X el is the elemental abundance relative to hydrogen; /and g are the oscillator strength and average Gaunt factor for the resonance fine transition; 0, is the relative population of the ground state of the ion; a = A2\I(A 1X + C 21 ) is the probability of an excitation leading to a photon; <ï> ion ( T) is the fraction of the given element in the ionization stage responsible for the resonance fine; T is the temperature in K; and n e and n h are the number densities of electrons and hydrogen nuclei, respectively. The form for the collisional rate incorporated in equation 28is that of Cochrane & McWhirter (1983) or Allen (1976, p. 42).…”

“…Abundances X el of carbon, silicon, and nitrogen relative to hydrogen are taken to be the solar photospheric values of 4.65 X 10 -4 , 3.69 X 10 -5 , and 8.30 X 10" 5 , respectively (Meyer 1985). To determine the relevant ion population $ ion of each element, we used the ionization equilibrium tables in Arnaud & Rothenflug ( 1985 ), together with a cubic spline interpolation to determine the dependence of log population on log T. For the C rv and N v lines, we use oscillator strengths of /= 0.286 and / = 0.235, respectively, and Gaunt factors g = 0.70 and g = 0.80, respectively (Cochrane & McWhirter 1983). For the Si m line, we use /= 1.70 and g = 0.34 (Dupree 1972), and for Si rv, /= 0.719 (Flower & Nussbaumer 1975) and g = 0.55 (Dupree 1972 ).…”

An elegant method to estimate helium-like ion densities from visible and VUV plasma spectroscopy

“…The excitation rate coefficients for oxygen have been calculated from the cross section results of Zhang et al (1990). The data for carbon have been taken from Cochrane and McWhirter (1983). Since the 3 p 2 P level is close to the ionization threshold, redistribution processes affect the effective excitation to 3p2P0 and the ionization so that the density dependence of EL^ and S are comparable.…”