Excitation of Metastable Levels in Low Density Nebular Plasmas: [SII] and [ArIV].

Diagnostics of low density nebular plasma by means of its forbidden line spectrum require not only a knowledge of certain intensity ratios as a function of its density and temperature but also a knowledge of the occupation numbers of relevant ionic levels. We present the necessary data for important levels of ionized sulfur and triply ionized argon.