Ab initio study of He(1 S )+Cl2( X  1Σ g ,3Π u ) potential energy surfaces

The potential energy surface of the ground state He+Cl2(1Σg) is calculated by using the perturbation theory of intermolecular forces and supermolecular Mo/ller–Plesset perturbation theory approach. The potential energy surface of the first excited triplet He+Cl2(3Πu) was evaluated using the supermolecular unrestricted Mo/ller–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, De=45.1 cm−1, Re=4.25 Å) and to the T-shaped structure with He perpendicular to the molecular axis (a secondary minimum, De=40.8 cm−1, Re=3.5 Å). 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 Cl2(3Πu), 3A′ and 3A″, the same two isomers correspond to minima. Now, however, the T-shaped form is lower in energy. The 3A′ and 3A″ states correspond to (De,Re) of (19.9 cm−1, 3.75 Å) and (30.3 cm−1, 3.50 Å), respectively, whereas the linear form is characterized by (19.8 cm−1, 5.0 Å). The binding energy for the T form in the lower 3A″ state is in good agreement with the experimental value of Beneventi et al.