Rotational and Near-IR Spectra of PbF: Characterization of the Coupled X₁\,²Π1/2 and X₂\,²Π3/2 States

Observations of the rotational spectrum of lead monofluoride, PbF, have been extended up to transitions in the v = 7 level for ²⁰⁸PbF in the lowest X₁\,²Π1/2 state of the radical and v = 5 for the ²⁰⁷Pb and ²⁰⁶Pb isotopologues. The data also include a few measurements for ²⁰⁴PbF in v = 0. These new measurements have been combined with existing near-IR measurements of the X₂ - X₁ fine-structure transition and a simultaneous multi-isotope fit of the data to an effective isotope-independent ro-vibronic Hamiltonian has been carried out. The resulting parameters fully characterize the vibrational, rotational and hyperfine structure of the combined X₁ \, / \, X₂ state of the radical. A pair of opposite parity levels with total angular momentum quantum number, $F=1/2$, in the lowest rotational level, $J=1/2$ of \,are close in energy and their spacing decreases with vibrational excitation. The experimental results show the spacing decreases to less than 20 MHz at $v=7$ and 8. The experimental work is complemented by new ab initio calculations which support the results and allow predictions outside the experimental data range. The calculated radiative lifetimes of the relevant vibrationally excited states are of the order of 50 ms. This work was motivated by interest in using \, as a vehicle for future probes of the standard model of physics such as placing limits on the electron's electric dipole moment (), molecular charge-parity non-conservation and Born-Oppenheimer breakdown effects for example.