Ab-initio-based model for the charge transfer mechanisms in Ar++H2O collisions

The mechanism of the charge-transfer reaction between water molecules and a rgon ions is analysed using a two-dimensional model with the reaction and H OH bending coordinates, which maintains the C-2v symmetry of the reaction s ystem. Potential energy surfaces and nonadiabatic coupling matrix elements are computed using ab initio Moller-Plesset perturbation theory with multip le partitioning of the full Hamiltonian. Both ab initio calculations and se miclassical simulations of the vibrational distributions of the product H2O -((A) over tilde) ion indicate two possible charge-transfer mechanisms. The first involves a nonadiabatic transition induced by radial coupling, where as the second is governed by the curve crossing along the bending coordinat e. The coexistence of two mechanisms agrees qualitatively with that inferre d from the experimental measurements. (C) 2000 Elsevier Science B.V.