Interstellar Sulfur Hydride: A Search for the 111-Megahertz Lines

Similarities in the energy-level structure of the sulfur hydride radical and the hydroxyl radical suggest that sulfur hydride in the interstellar medium might be detectable because of a population inversion or anti-inversion similar to that of the hydroxyl radical. We have searched for the 111.54-megahertz transition [ F (total angular momentum quantum number) = 2 → 2] and for the 111.22-megahertz transition ( F = 1 → 1) in the galactic radio source W49, one of the brightest hydroxyl emission sources. No sulfur hydride emission lines with half-power widths of 130 hertz or greater were detected with the 1000-foot Arecibo antenna. The upper limits established with 100-hertz filters were 50 and 60 flux units (1 flux unit= 1026 watt meter-2 hertz-1), respectively, for the two lines.