The electronic structure of alkali bihalide molecules MX2. II. Excited states of LiF2, NaF2, LiCl2, and NaCl2 systems

Abstract

The low-lying valence excited states of the alkali bihalide molecules LiF2, NaF2, LiCl2, and NaCl2 are investigated in this research. This theoretical study is based on restricted-Hartree–Fock theory and employs large Gaussian basis sets in the calculations. This work complements previous work on the ground states of the same systems and uses identical methods and basis sets. A major focus is to examine the effect of the M+ counter ion on the vertical electronic transition energies between the ground state and the excited states of MX2 in comparison to parallel transitions in free X−2. We find the electronic transition energies in MX2 are very similar to parallel transitions in X−2 and that the M+ counterion has a small, but detectable, influence on the transition energy. A detailed comparative analysis is presented, i.e., the influence of M+, and pertinent geometric parameters, on the vertical electronic transitions from the isosceles triangle ground state (C2v) of MX2 systems. Only limited consideration is given to the detailed geometry of the excited state species, but it is apparent that a linear X––M––X structure is likely for certain excited states of MX2.