Potential energy hypersurface for the 1 2Π(1 2B1,1 2A′′) Rydberg state of the HeH2+ system

Abstract

The 1 ²Π(1 ²B₁,1 ²A^″) state of the helium dihydride ion HeH₂⁺ has been examined with ab initio electronic structure calculations. In particular, the potential energy hypersurface for this Rydberg state has been surveyed with an MCSCF/CI wavefunction involving orbital expansions in a double‐zeta plus double polarization plus diffuse one‐electron basis. The surface is essentially a trough falling monotonically from the atom–diatomic asymptote He(X ¹S) +H₂⁺(C ²Π_u), through the triatomic region, to the asymptote HeH(1 ²Π)+H⁺. At some triatomic geometries the 1 ²Π state is one of the lowest excited electronic states of this molecular ion system. The results obtained here are the first reported explicit treatment of HeH₂⁺ Rydberg states, and will contribute towards the elucidation of the manifolds of the lowest two Rydberg series arising from the (HeH,H)⁺ excited asymptotes.