The pure rotational spectrum of KH (X¹Σ⁺): Quadrupole coupling in alkali hydrides

The pure rotational spectrum of KH (X¹Σ⁺) has been recorded using millimeter/submillimeter direct absorption techniques. The species was created by the reaction of potassium vapor and H₂ in a d.c. discharge. The J = 0 → 1 and 1 → 2 transitions were measured near 202 and 404 GHz, and electric quadrupole splittings were resolved in the J = 0 → 1 line and partly resolved in the other transition. For comparison, the quadrupole structure in the J = 0 → 1 transition of NaH (X¹Σ⁺) was also recorded. Very accurate rotational and quadrupole hyperfine constants were determined for KH, and eqQ was established for NaH as well. Diatomic hydride species are the building blocks for interstellar chemistry in both dense and diffuse gas. Knowledge of the rest frequencies of KH will enable its detection in such objects.