AB-INITIO STUDY OF HE(S-1)+CL-2(X(1)SIGMA(G),(3)PI(U)) POTENTIAL-ENERGY SURFACES

The potential energy surface of the ground state He+ Cl-2((1) Sigma(g)) is calculated by using the perturbation theory of intermolecular forces and supermolecular Moller-Plesset perturbation theory approach. The potential energy surface of the first excited triplet He+Cl-2((3) Pi(u)) was evaluated using the supermolecular unrestricted Moller-Plesset perturbation theory approach. In the ground state two stable isomers are found which correspond to the linear He-Cl-Cl structure (a primary minimum, D-e=45.1 cm(-1), R(e)=4.25 Angstrom) and to the T-shaped structure with He perpendicular to the molecular axis (a secondary minimum, D-e=40.8 cm(-1), R(e)=3.5 Angstrom). The small difference between these geometries is mainly due to the induction effect which is larger for the linear form. The results obtained for the T-shaped minimum are in good agreement with the excitation spectroscopy experiments which observed only the T-shaped form [Beneventi et al., J. Chem. Phys. 98, 178 (1993)]. In the lowest triplet states correlating with Cl-2((3) Pi(u)), (3)A' and (3)A'', the same two isomers correspond to minima. Now, how-ever, the T-shaped form is lower in energy. The (3)A' and (3)A'' states correspond to (D-e,R(e)) of (19.9 cm(-1), 3.75 Angstrom) and (30.3 cm(-1), 3.50 Angstrom), respectively, whereas the linear form is characterized by (19.8 cm(-1), 5.0 Angstrom). The binding energy for the T form in the lower (3)A'' State is in good agreement with the experimental value of Beneventi et al.