Excited states of PbF: a four-component relativistic study.

The electronic states of lead monofluoride (PbF) are studied from the (Pb 6s)(2) (F 2p-pi)(4) (F 2p-sigma)(2) (Pb 6p-pi)(1) X(1) ground state up to the F state, using the four-component relativistic configuration interaction and Fock-space coupled-cluster singles and doubles methods. Difficulties arising from the valence-Rydberg mixing are overcome by using a flexible basis set including Rydberg-type diffuse functions and by large-scale correlation calculations. The excited states are successfully characterized with the help of computed transition dipole moments. The three lowest-lying states (X(1), X(2), and A) are confirmed to be valence states arising from the (Pb 6p) spinors. The B state is assigned to the lowest Rydberg state (Omega=1/2), represented by a single excitation from the (Pb 6p-pi) spinor to the (F 3s) Rydberg spinor. Its calculated excitation energy (4.30 eV) is comparable to the observed one (4.42 eV). The C state is a multiconfigurational valence state whose dominant configuration is represented by (Pb 6s)(2) (F 2p-pi)(4) (F 2p-sigma)(1) (Pb 6p-pi)(2). Its calculated excitation energy (4.71 eV) is in good agreement with experiment (4.72 eV). The remaining D, E, and F states are assigned as Rydberg states. The calculated ionization potential (7.44 eV) is also close to the value (7.55 eV) determined recently by multiphoton ionization experiments.