THE OPTICALLY THICK C-III SPECTRUM .1. TERM POPULATIONS AND MULTIPLET INTENSITIES AT LOWER OPTICAL DEPTHS

The C III spectrum is studied quantitatively under both optically thin and optically thick conditions, yielding term populations and line/multiplet intensities for column lengths from zero to 10(18) cm-2. The roles of escape probabilities and line profiles in the calculation are discussed in some detail. It is shown that use of the fully integrated escape factor, rather than the more appropriate monodirectional escape probability, can lead to appreciable errors in calculated intensities. The results for populations and intensities allow us to identify two unassigned features in the solar EUV spectrum of Vernazza & Reeves as C III multiplets, and to establish that an unidentified infrared solar feature at 8500.32 angstrom, seen in both absorption (Fraunhofer) and emission (chromospheric) spectra, is the C III transition 2s3s(1S)-2s3p(1P). In addition to the principal results we tabulate Voigt parameters for the C III lines and multiplets, obtained by a modified semiclassical method. We also propose and use a new, unambiguous notation for the numerous line ratios present in a typical spectrum, which we believe to be an improvement over present arbitrary notations.