Excited-state populations of singly ionized sulphur in gaseous nebulae
Weisheit, Jon Carleton
Clayton, Donald D.
Master of Science
A detailed study is made of the populations of singly ionized sulphur (SII) metastable states. These four states, 2D3/2, 2D5/2, 2P1/2, 2P3/2, and the ground state, 4S3/2, arise from a 3p3 electron configuration. Rates of populating the five states by collisional and radiative processes prominent in gaseous nebulae are examined. Inelastic electron-SII collision cross sections are estimated using a scaling technique suggested by Osterbrock (1965). The quantum defect method is used to calculate the photoionization cross sections of neutral and singly ionized sulphur. With these and related data, population rate equations for all SII (3p3) states are solved for numerous combinations of electron number densities and temperatures in the ranges 500<=Ne (cm-3)<10^9; and 5,000<Te(°K)<30,000. The number density ratio N(2D5/2)/N(2D3/2), as a function of Ne and Te, is related to the intensity ratio of the forbidden transitions between 2D5/2 - 4S3/2(?6717) and 2D3/2-4S3/2(?6731). The results are used in conjunction with the intensity ratios observed in gaseous nebulae to obtain electron number densities in the regions of SII emission. Finally, additional uses of SII spectra are noted.