With the Cologne terahertz spectrometer, the lower J rotational spectrum of the SD radical has been detected for the first time in the laboratory, in analogy with our previous recent detection of the SH radical. The radicals were produced by discharging H2S (D2S) buffered with He and H2 (D2). The spectra were analyzed employing Hund's case (a) coupling scheme. The SH radical has an inverted 2Π state, as does OH, so that the 2Π3/2 state is the energetically lower one. For both SH and SD, the rotational and centrifugal distortion constants, the spin orbit, the spin rotation, the Λ-doublet, and the hyperfine interaction constants were determined by combining the newly acquired experimental data with those available in the literature. The rotational constant B0 for SD is 146885.297 (26) MHz, while our latest value for SH is 283587.62 (12) MHz (the values in parentheses denote the calculated uncertainty obtained by a least-squares fit). The SH radical and its deuterated isotopomer may be detectable in interstellar space because of the high cosmic abundance of elemental sulfur and the high abundance of HS. Since the 2Π1/2 electronic state lies nearly 400 cm–1 above the 2Π3/2 state, the rotational transitions in the 2Π1/2 state should only be detectable in hot interstellar sources, and they are potentially good indicators of star formation.
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