Theory and application of photoelectron spectroscopy. 100. Variable-temperature photoelectron spectral study of 1,3-dithiol-2-one and 4,5-disubstituted 1,3-dithiol-2-ones. Thermal generation of 1,2-dithiete, 3,4-disubstituted 1,2-dithietes, and dialkyl tetrathiooxalates