Potential Energy Curves for the ¹Σ⁺ and ^1,3Π States of CO

Abstract

Ab initio potential energy curves of CO are calculated to address a number of problems remaining in the interpretation of the experimental VUV absorption spectra. The calculations are of the type SCF MRSD-CI. We employed the aug-cc-pVQZ basis set for both carbon and oxygen, augmented with twelve diffuse functions, of s-, p-, and d-type, located on both atomic centers. We focus on the energy region 85 000 cm−1 < E < 110 000 cm−1 characterized by strong interactions between Rydberg and valence states. In this work we deal specifically with the 1Σ+, 1Π, and 3Π states lying in this region. Some of the relevant findings are as follows: The minimum of the C′1Σ+ valence state is found 1920 cm−1 above the value inferred from an extrapolation of experimental data. A new 1Π valence state, labeled E′, is found to perturb strongly the (X2Σ+)3pπ E1Π Rydberg state. The electrostatic perturbation of the (X2Σ+)3pπ c3Π Rydberg state by the k3Π valence state is confirmed. The energy position of the (A2Π)3sσ 3Π state, the triplet companion of the W1Π Rydberg state, is predicted.