CEPA calculations of potential energy surfaces for open-shell systems. Pt. 7

Potential energy surfaces for the collision of He atoms with NH radicals in the electronically excited A3Π 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 distances R and HNHe angles γ was chosen to be 4.0≤R≤8.0α0, 0≤γ≤1800. The doubly degenerate NH(A3Π) state is split upon approach of He into two components, 13 A' and 23A'', which remain degenerate only for collinear geometries. The resulting two potential surfaces VA' and VA'' are essentially repulsive with shallow van der Waals minima at large distances. An expansion of the sum and the difference potential, VA'+VA'' and VA'-VA'', respectively, in terms of Legendre polynomials shows that the anisotropic components V10(R) and V22(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(A3Π)-He collisions. This is in accord with recent experiments of Kaes and Stuhl. (orig.)