Fourier transform microwave spectroscopy of the $Σ+2$ ground states of YbX (X=F, Cl, Br): Characterization of hyperfine effects and determination of the molecular geometries

The pure rotational spectra of low lying $v=0$ and 1 vibrational levels of the $Σ+2$ ground states of several isotopomers of YbF, YbCl, and YbBr were recorded using a pulsed jet cavity Fourier transform microwave spectrometer. Through least squares fits, parameters describing rotational, fine, and hyperfine effects (such as Fermi-contact, dipole–dipole coupling, and nuclear spin-rotation coupling) are presented. Trends in this halide series show an increasing amount of unpaired spin-density on the halide nucleus with increasing halide atomic number, which is consistent with results obtained in previous studies of the calcium analogs. Effective equilibrium bond lengths for ¹⁷⁴YbF and ¹⁷⁴Yb³⁵Cl have been determined to be 2.016 514(1) Å and 2.488 285(1) Å, respectively. The spectra of the two major bromine isotopomers, ⁷⁹Yb¹⁷⁴Br and ⁸¹Yb¹⁷⁴Br, were studied only in the $v=0$ level, and the derived values of $r0$ are 2.647 364 5(2) Å and 2.647 347 6(2) Å, respectively. The uncertainties in the equilibrium bond lengths of YbF and YbCl are discussed in terms of the measurement accuracies of the spectra and of the Born–Oppenheimer approximation.