Ab initio calculation of the OH (X 2Pigrec, A 2Sigma+)+Ar potential energy surfaces and quantum scattering studies of rotational energy transfer in the OH (A 2Sigma+) state

The potential energy surfaces of OH+Ar, which correlate asymptotically with OH(X 2?)+Ar(1S) and OH(A 2?+)+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.