Abstract
The electronic structure of the molecules LaF⁺ and LaF was studied using frozen-core four-component multiconfigurational quasidegenerate perturbation theory. To obtain proper excitation energies for LaF⁺, it was essential to include electronic correlations between the outermost valence electrons (4f, 5d, and 6s) and ionic core electrons composed of (4s, 4p, 4d, 5s, and 5p). The lowest-lying 16 excited states were examined for LaF⁺, and the lowest 30 states were examined for LaF. The excitation energies calculated for LaF⁺ agree with the available experimental values, as well as with values from ligand field theory. Errors are within 0.4 eV; for example, the highest observed state ²Pi is 3.77 eV above the ground state, and the present value is 4.09 eV. For LaF, agreement between the experimental and theoretical state assignments and between the experimental and calculated excitation energies was generally good, except for the electron configurations of certain states. Errors are within 0.4 eV except for a single anomaly; for example, the highest observed excited-state discussed in this work is 2.80 eV above the ground state, and the present value is 2.42 eV. We discuss the characteristics of the bonding in LaF⁺ and LaF.