Direct Absorption Spectroscopy at 1.1 THz: New Measurements of CrH (X⁶Σ⁺)

The pure rotational spectrum of the ⁵²CrH radical in its X⁶Σ⁺ state at 1.1 THz has been recorded with direct absorption methods using a cascaded chain multiplier source. The N = 3 ←2 transition has been measured in multiple spin components for ΔJ = 0, ± 1, and the proton hyperfine structure resolved. The transition was also recorded for the ⁵³Cr isotopologue in natural abundance and both the proton and ⁵³Cr hyperfine splittings measured─the first observation of this species by rotational spectroscopy. The data, including previously measured N = 1 ← 0 and 2 ← 1 frequencies for the main isotopologue, were analyzed with a Hund’s case (b) Hamiltonian that included third-order spin-rotation and fourth-order spin–spin terms. Rotational, fine structure, and proton hyperfine constants were determined for CrH, as well as ⁵³Cr hyperfine parameters for ⁵³CrH. In particular, the higher-order parameters γ_s and θ were established to a high degree of accuracy. The hyperfine constants for both nuclei are consistent with the unpaired electrons being principally located on the chromium nucleus. The Fermi contact term b_F for the ⁵³Cr nucleus suggests that the unpaired σ electron occupies an orbital mostly 3d in character.