Accurate Ionization Energies for MF and Characterization of the Low-Energy States of MF+ (M = Sc, Y, La)

Abstract A subset of monovalent metal fluorides have been identified as promising candidates for precision spectroscopic measurements that probe physics beyond the standard model. The interpretation of the precision data will require parameters that can only be derived from validated high-level electronic structure models. Recent calculations have focused on the use of ionization energies (IEs) as a means to evaluate the quality of the computational predictions. However, validation depends on the availability of accurate experimental data. In this study we report IE values for ScF, YF, and LaF. These were previously known with errors of ±0.3 eV. The present measurements yielded IEs of 6.6121(12), 6.6937(12), and 5.6104(12) eV, respectively. The two-photon ionization technique used for these measurements also permits the observation of the low-energy states of the MF+ cations. The vibronic structures established that the ground states were ScF+ X2Δ, YF+ X2Σ+, and LaF+ X2Δ. The IEs for ScF and LaF predicted by a recent computational study were significantly higher than the measured values. This discrepancy was traced to the fact that MF+ 2Σ+ ground states had been assumed. The computed IE for YF, where the cation does have a 2Σ+ ground state, was in good agreement with experiment.