Rovibronic spectroscopy of the van der Waals complex He–HCl⁺

Abstract

The potential energy surfaces and the calculated rovibronic spectrum of the electronic ground state of the van der Waals He–HCl⁺ cation are presented. The system is in a X ²Π electronic state at linearity, which splits into an A′ and an A″ state upon bending, leading to a Renner–Teller effect. Three-dimensional potential energy surfaces have been determined using the partially spin-restricted open-shell single and double excitation coupled cluster method with perturbative triples [RCCSD(T)]. The absolute minimum of a two-dimensional surface with the diatom bond length r fixed at r _e = 2.489 a ₀ is located at the linear He–HCl⁺ geometry with a van der Waals bond length R of 5.98 a ₀ and D _e ≃ 300 cm⁻¹. The minimum in the full three-dimensional potential occurs for a slightly larger value of r: 2.492 a ₀. The rovibronic levels of the He–HCl⁺ complex have been computed by a variational method for total angular momenta J =1/2, 3/2, 5/2, 7/2 and 9/2. The binding energy D ₀ is calculated as 161.5 cm⁻¹ using the two-dimensional potential energy surfaces with r frozen at r _e and as 163.5 cm⁻¹ in full three-dimensional calculations. Owing to the large and negative value of the spin–orbit parameter in HCl⁺ (A _SO = −648.13 cm⁻¹), all the considered rovibronic states correspond to the |Ω| = 3/2 spin–orbit component of HCl⁺. The nuclear wave functions of the complex could be interpreted using the model of a slightly hindered diatomic rotor. The energy level pattern and wave functions have been compared with the more floppy Ar–OH complex on the one hand and the more rigid He–HF⁺ system on the other. The anisotropy of the potential energy surface of the He–HCl⁺ complex is intermediate between these two cases and the rovibronic states reflect this property.