A CASSCF/ACPF study of spectroscopic properties of FeS and FeS− and the photoelectron spectrum of FeS−

Various electronic states of FeS and FeS− have been studied theoretically. The ground states are 5Σ+ and 6Δ, respectively, with re=201 and 216 pm, we=536 and 450 cm−1 and D0=3.05 and 2.66 eV. The electron affinity of FeS is 1.63 eV. An assignment of the photoelectron spectrum of FeS− is given. The calculation of bond dissociation energies requires large basis sets, while for energy differences between different electronic states smaller ones are sufficient. Density functional theory calculations perform well for spectroscopic constants but less well for energy differences between different electronic states.