CEPA calculations of potential energy surfaces for open-shell systems

Potential energy surfaces for the collision of He atoms with NH radicals in the electronically excitedA ³ Π state have been calculated using quantum chemical ab initio methods. The NH distance was kept fixed to its equilibrium value, the range of the HN-He distancesR and HNHe angles γ was chosen to be 4.0≦R≦8.0a ₀, 0≦γ≦180°. The doubly degenerate NH(A ³ Π) state is split upon approach of He into two components, 1³ A′ and 2³ A″, which remain degenerate only for collinear geometries. The resulting two potential surfacesV _A′ andV _A″ are essentially repulsive with shallow van der Waals minima at large distances. An expansion of the sum and the difference potential,V _A′ +V _A″ andV _A′ ?V _A″, respectively, in terms of Legendre polynomials shows that the anisotropic componentsV ₁₀(R) andV ₂₂(R) which mainly govern rotational transitions and Λ-doublet mixing are of the same size. It is therefore expected that these processes as well as fine-structure transitions are similarly probable in NH(A ³ Π)-He collisions. This is in accord with recent experiments of Kaes and Stuhl.