A b i n i t i o calculation of the OH (X 2Π, A 2Σ+)+Ar potential energy surfaces and quantum scattering studies of rotational energy transfer in the OH (A 2Σ+) state

Abstract

The potential energy surfaces of OH+Ar, which correlate asymptotically with OH(X 2Π)+Ar(1S) and OH(A 2Σ+)+Ar(1S), have been calculated using the coupled electron pair approximation (CEPA) and a very large basis set. The OH–Ar van der Waals complex is found to be bound by about 100 cm−1 in the electronic ground state. In agreement with several recent experimental studies the first excited state is found to be much more stable. The A state potential energy surface has two minima at collinear geometries which correspond to isomeric OH–Ar and Ar–OH structures. The dissociation energies De are calculated to be 1100 and 1000 cm−1, respectively; both forms are separated by a barrier of about 1000 cm−1. The equilibrium distances for OH–Ar and Ar–OH are calculated to be 2.9 and 2.2 Å, respectively, relative to the center of mass of OH. In order to investigate the nature of the strong binding in the A state, we have calculated accurate dipole and quadrupole moments as well as dipole and quadrupole polarizabilities for the X and A states of the OH radical and for the Ar atom. These data are used to estimate the contributions of induction and dispersion forces to the long-range OH–Ar potential. The calculated potential energy surfaces have been fitted to an analytical function and used in quantum scattering calculations for collision induced rotational energy transfer in the A state of OH. From the integral cross sections rate constants have been evaluated as a function of the temperature. The theoretical rate constants are considerably larger than the corresponding experimental values of Lengel and Crosley [J. Chem. Phys. 67, 2085 (1977)], but in good agreement with recent measurements of Jörg, Meier, and Kohse-Höinghaus [J. Chem. Phys. (submitted)]. Our potential energy surface has also been used to calculate the bound rovibrational levels of the OH–Ar complex.