Ab initio calculations of the interaction of He with the B-3 Pi(0u(+)) state of Cl-2 as a function of the Cl-2 internuclear separation

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 unrestricted coupled cluster singles, doubles, and noniterative triples (UCCSD(t)) calculations were performed for several points on the B electronic state surface and were used to scale the UMP-4 points. Exp-6 type two center potential energy functions were fitted to the modified UMP-4 points (B state) to construct an analytical three-dimensional potential energy surface. An r (Cl-Cl separation) dependence was incorporated in the B state potential energy surface to allow the calculation of HeCl2 properties in different vibrational states so that vibrational predissociation rates could be calculated. Excitation spectra, predissociation 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 experimental one, and the calculated blueshifts for nu=8, 10, 12 show the right trend when compared to the experimental findings, i.e., the blueshifts 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 the X state. The calculated vibrational predissociation lifetimes tau are in good agreement with the experiment, as are the rotational product distributions for nu=8, 10, and 12. (C) 1997 American Institute of Physics.