Ab initio study of He(1S)+Cl2(X 1Σg,3Πu) potential energy surfaces

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Abstract
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.