Spectroscopic characterization of buffer-gas-cooled lead monofluoride molecules in the B ²Σ⁺(υ’ = 0) ← X₁ ²Π_1/2(υ = 0) transition

Establishing a nonzero measurement of the electron Electric Dipole Moment (eEDM) has long been a fundamental pursuit in atomic, molecular and optical physics, offering possible insights into new physics beyond the Standard Model. In this regard, lead monofluoride (PbF) has emerged as a potential candidate for measuring eEDM primarily due to its suitable properties such as the strong internal effective electric field, and eEDM-sensitive ground state with large Ω-doubling and small magnetic g factor. In the present work, we realized the production of a buffer-gas-cooled PbF molecular beam and characterized its high-resolution spectroscopy in the B ²Σ⁺(υ’=0) ← X₁ ²Π_1/2(υ = 0) transition, including both direct absorption and laser-induced fluorescence spectroscopy. A highly concentrated beam of PbF molecules is obtained with a central forward velocity of 223 ± 17 m/s, while 81, 66 and 24 hyperfine-structure-resolved spectral lines with a frequency accuracy of 40 MHz have been assigned respectively for ²⁰⁸PbF, ²⁰⁷PbF and ²⁰⁶PbF isotopologues. The hyperfine constants due to the ¹⁹F nucleus (A_∥ and A_⊥) of the B state are reported for the first time, and those of the ²⁰⁷Pb nucleus have been also updated. Such a cryogenic molecular beam of PbF in association with its hyperfine-structure-resolved spectral atlas of the B ²Σ⁺(υ’=0) ← X₁ ²Π_1/2(υ = 0) transition will be essential in developing sensitive detection schemes towards the eEDM measurement.