The infinite-order sudden approximation for collisions involving molecules in Π electronic states: A new derivation and calculations of rotationally inelastic cross sections for NO(X 2Π)+He and Ar

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A new derivation is presented of the infinite-order sudden (IOS) approximation for rotationally inelastic collisions of a diatomic molecule in a Π electronic state with a closed-shell atom. This derivation clearly demonstrates the connection between the two sudden S functions for scattering off the adiabatic potential surfaces of A′ and A″ symmetry, which would arise from an ab initio calculation on an atom+Π-state molecule system, and the S-matrix elements in a diabatic basis, which are required in the quantum treatment of the collision dynamics. Coupled-states and IOS calculations were carried out for collisions of NO(X 2Π) with He and Ar, based on the electron gas potential surface of Nielson, Parker, and Pack [J. Chem. Phys. 66, 1396 (1977)], at total energies of 63, 150, and 300 meV. The IOS approximation is not reliable for collisions of NO with Ar, even at the highest collision energy considered here. However, for collisions with He at 150 and 300 meV, the IOS approximation is nearly quantitative for transitions both within and between the Ω=1/2 and Ω=3/2 manifolds.