Fine-Structure Transitions

Abstract

We calculate collision cross-sections and photon transition probabilities for many cases of practical interest. Using the calculated rates, we show how the fine structure of absorption lines can be used as an indicator of the density of the absorbing medium or the ambient intensity of a strong photon flux. For densities much greater than a critical value, ~ or for very high photon fluxes, the fine-structure levels of the ground state of a given ion are populated according to a Boltzmann distribution (which for most practical cases means according to the ratio of degeneracies) For densities much less than Ncr and moderate photon fluxes, only the lowest fine structure state is populated The value of N0~ is calculated for the most abundant ions that have resonant absorption lines with wavelengths greater than 950 A by evaluating the rates of the photon and collisional transitions among the fine structure levels. Cross-sec- tions for collisional excitation by protons have been calculated by numerical integration of the relevant time-dependent Schroedinger equation for six ions of interest and these detailed calculations are used to obtain accurate estimates of the proton-excitation cross-sections for a number of other cases. The prob- ability of exciting fine-structure levels of both neutral and charged ions by collisions with hydrogen atoms has been estimated. The rates of magnetic-dipole photon decays, as well as cross-sections for colli- sional excitation of fine-structure levels by electrons, have been calculated from previously published results. Applications are made to problems involving the interstellar medium, circumstellar envelopes of giant stars, the solar corona, planetary nebulae, the intergalactic medium, and quasi-stellar source