The isotopic dependence of the spin-rotation interaction: the rotational spectrum of cadmium hydride in its XΣ state

Far-infrared rotational transitions within the X²Σ⁺ (v=0) state of cadmium hydride (CdH) were recorded over the range N″=2-17 for 12 different isotopomers, using the technique of tunable far-infrared (TuFIR) spectroscopy. The molecule was made by heating cadmium metal in the presence of a DC electric discharge in hydrogen. Fine structure arising from the electron spin-rotation interaction and hyperfine structure from the ¹¹¹Cd, ¹¹³Cd, and ¹H nuclei were resolved and analyzed. All of the isotopic data were fitted together using a Hamiltonian containing mass-independent, Dunham-type rotational parameters U_kl and small correction terms Δ_kl described by Watson J. Mol. Spectrosc. 80 (1980) 411]. The spin-rotation interaction was modeled in an analogous way using Dunham-like U_γ,kl parameters, and fitting its isotopic dependence properly required the use of four Δ_γ,kl correction terms.