AB-INITIO CALCULATIONS OF THE INTERACTION OF HE WITH THE B(3)PI(0U(-2AS A FUNCTION OF THE CL-2 INTERNUCLEAR SEPARATION()) STATE OF CL)

Ab initio calculations using unrestricted Moller-Plesset perturbation theory to fourth order (UMP-4) were carried out for the interaction of He with the B (3) Pi(0u+) state of Cl-2, Also, more reliable unrestri cted coupled cluster singles, doubles, and noniterative triples (UCCSD (t)) calculations were performed for several points on the B electroni c state surface and were used to scale the UMP-4 points. Exp-6 type tw o center potential energy functions were fitted to the modified UMP-4 points (B state) to construct an analytical three-dimensional potentia l energy surface. An r (Cl-Cl separation) dependence was incorporated in the B state potential energy surface to allow the calculation of He Cl2 properties in different vibrational states so that vibrational pre dissociation rates could be calculated. Excitation spectra, predissoci ation lifetimes, and rotational product distributions were calculated and compared to the available experimental data. It was found that the calculated B<--X, 8<--0 spectrum is in good agreement with the experi mental one, and the calculated blueshifts for nu=8, 10, 12 show the ri ght trend when compared to the experimental findings, i.e., the bluesh ifts get larger with increasing v. The blueshift values are generally too small which suggests that the nb initio calculations underestimate the van der Waals interactions in the B state less than they do in th e X state. The calculated vibrational predissociation lifetimes tau ar e in good agreement with the experiment, as are the rotational product distributions for nu=8, 10, and 12. (C) 1997 American Institute of Ph ysics.