Configuration interaction studies on low-lying valence and Rydberg states of PO

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Abstract
CI studies with a double-zeta plus polarization plus diffuse basis set were performed on the X 2Π, a4Π, B 2Σ+, B′ 2Π, b 4Σ−, D′ 2Π, A 2Σ+, and C′ 2Δ states of PO over a wide range of internuclear separations. Spectroscopic constants, as well as adiabatic excitation energies and dissociation energies were calculated, and are in good agreement with experimental values. Allowing for crossing of some 2Π and 2Σ+ states improves their calculated spectroscopic constants and identifies short sections of the D 2Π, F 2Σ+, and A′ 2Σ+ (repulsive) states. Particular emphasis was placed on the nature of the B 2Σ+ state, which is shown to require diffuse orbitals for a proper description, although it is not a Rydberg state. The valence-isoelectronic molecule NO does not have an equivalent state.