On the perturbation of the B̃ 2A′ state of HCO by the 1 4A″ and 1 4A′ states: Surfaces of intersection and spin–orbit interactions

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Abstract
The excited B̃ 2A’ state of HCO is studied using multiconfiguration self‐consistent field/configuration interaction wave functions of 300 000 to 1 400 000 terms. Equilibrium structures for the X̃ 2A’, B̃ 2A’, 1 4A‘, and 1 4A’ states are determined as is the X̃ 2A’–B̃ 2A’ electronic transition dipole moment. This dipole transition moment vector is found to make an angle of ∼+34° with the inertial a axis of the B̃ 2A’ state. The possibility of spin–orbit induced perturbation of the low‐lying vibrational levels of the B̃ 2A’ state attributable to the 1 4A‘ or 1 4A’ states is considered. The minimum energy points on the surface of intersection of the B̃ 2A’ and 1 4A‘ states, denoted MECP(B̃ 2A’–1 4A‘), and of the B̃ 2A’ and 1 4A’ states, denoted MECP(B̃ 2A’–1 4A’), are determined. MECP(B̃ 2A’–1 4A‘) is located very close to the 1 4A‘ equilibrium structure and is only ∼1500 cm−1 above the B̃ 2A’ minimum. At MECP(B̃ 2A’–1 4A‘) the parallel and perpendicular components of the B̃ 2A’∼1 4A‘ spin–orbit coupling, based on the microscopic Breit–Pauli operator, are approximately 50 and 4 cm−1, respectively. No barrier was found separating the B̃ 2A’ equilibrium structure and MECP(B̃ 2A’–4A‘). MECP(B̃ 2A’–1 4A’) is close to the 1 4A’ minimum and is placed ∼2400 cm−1 above the minimum of the B̃ 2A’ state. However, the B̃ 2A’∼1 4A’ spin–orbit interaction, a perpendicular interaction, is only ∼1 cm−1. Additional points on the B̃ 2A’–1 4A‘ surface of intersection and the corresponding B̃ 2A’∼1 4A‘ spin–orbit interactions are determined.

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