Characterizing the fundamental bending vibration of a linear polyatomic molecule for symmetry violation searches

Polyatomic molecules have been identified as sensitive probes of charge-parity violating and parity violating physics beyond the Standard Model (BSM). For example, many linear triatomic molecules are both laser-coolable and have parity doublets in the ground electronic X-2 Sigma(+)(010) state arising from the bending vibration, both features that can greatly aid BSM searches. Understanding the X-2 Sigma(+)(010) state is a crucial prerequisite to precision measurements with linear polyatomic molecules. Here, we characterize the fundamental bending vibration of (YbOH)-Yb-174 using high-resolution optical spectroscopy on the nominally forbidden X-2 Sigma(+ )(010) -> A(2)Pi(1/2)(000) transition at 588 nm. We assign 39 transitions originating from the lowest rotational levels of the X-2 Sigma(+ )(010) state, and accurately model the state's structure with an effective Hamiltonian using best-fit parameters. Additionally, we perform Stark and Zeeman spectroscopy on the X-2 Sigma(+ )(010) state and fit the molecule-frame dipole moment to D-mol = 2.16(1) D and the effective electron g-factor to g(S) = 2.07(2). Further, we use an empirical model to explain observed anomalous line intensities in terms of interference from spin-orbit and vibronic perturbations in the excited A(2)Pi(1/2)(000) state. Our work is an essential step toward searches for BSM physics in YbOH and other linear polyatomic molecules.