Direct measurement of the N = 0 → 1 Transition of SH⁺ (X ³σ^-) by submillimeter velocity modulation spectroscopy

The N = 0 → 1 transition of SH⁺ in its ³σ^- ground electronic state has been recorded using submillimeter direct absorption/velocity modulation techniques. This molecular ion was created in an alternating current (AC) discharge of H₂S and argon, or CH₃SH in argon. Four hyperfine lines of the N = 0 → 1 transition were measured: three near 526 GHz and one near 346 GHz, arising from the J = 1 → 2 and J = 1 → 0 spin components, respectively; these four lines contribute 64% of the total intensity of the N = 0 → 1 transition. The measurements have been used to refine the molecular constants of SH ⁺, in particular the fine-structure parameters. Both the transition frequencies and the derived constants are in good agreement with previous laser magnetic resonance data. This study has resulted in very accurate rest frequencies for SH⁺, which is predicted to be an abundant species in photon-dominated regions.