Pure rotational spectra of PbSe and PbTe: potential function, Born–Oppenheimer breakdown, field shift effect and magnetic shielding

The pure rotational spectra of 41 isotopic species of PbSe and PbTe have been measured in their X¹Σ⁺ electronic state with a resonator pulsed-jet Fourier transform microwave spectrometer. The molecules were prepared by laser ablation of suitable target rods and stabilised in supersonic jets of noble gas. Global multi-isotopologue analyses yielded spectroscopic Dunham parameters Y₀₁, Y₁₁, Y₂₁, Y₃₁, Y₀₂, and Y₁₂ for both species, as well as effective Born–Oppenheimer breakdown (BOB) coefficients δ₀₁ for Pb, Se and Te. Unusual large values of the BOB parameters for Pb have been rationalized in terms of finite nuclear size (field shift) effect. A direct fit of the same data sets to an appropriate radial Hamiltonian yielded analytic potential energy functions and BOB radial functions for the X¹Σ⁺ electronic state of both PbSe and PbTe. Additionally, the magnetic hyperfine interactions produced by the uneven mass number A nuclei ²⁰⁷Pb, ⁷⁷Se, ¹²³Te, and ¹²⁵Te were observed, yielding first determinations of the corresponding nuclear spin–rotation coupling constants.