Metastability in the sulphur molecule S2+2 and S3+2 cations. A theoretical study

Abstract

Complete active space SCF (CASSCF) calculations followed by second order perturbation calculations (CASPT2) are performed for the ground state and for some low lying excited states of the S2 molecule di-cation and tri-cation. Spectroscopic values of the S2 parent molecule and its ions, S2 +, S2 2+, and S2 3+ are calculated from the potential energy curves. The performance of the active space selected for CASPT2 and the ANO (atomic natural orbitals) basis set is verified by coupled cluster CCSD(T) calculations for ionization potential and spectroscopic constants of the S2 molecule and its S2 + cation as well as for ionization potentials of the sulphur atom. Due to a multireference character of the potential energy curve of the doubly and triply charged S2 cations the performance of CCSD(T) deteriorates at longer distances from the minima and is thus not applicable over the whole surface. The stability of the title ions is discussed in terms of the barrier height and half-widths together with the tunnelling lifetimes for the metastable electronic states. It is shown that the metastable S2 2+ and S2 3+ cations are the ground states for the respective ions (the closed shell 1Σg + state and the 2Π state, respectively) and that repulsive curves are far from their low vibrational levels. That means that the depletion mechanism through predissociation is improbable. It is shown that especially the S2 3+ ion is able to release a considerable amount of energy, 6.97eV, after its decay. For the S2 2+ ion it is 1.24eV. The barrier for dissociation is 2.73 eV for S2 2+ and 0.66eV for S2 3+. The lifetimes for both metastable cations are predicted for a few vibrational levels. For both ions, S2 2+ and S2 3+, we have also detected additional (excited) metastable states: The 3Σu +, 3Δu and 3Πg states for S2 2+ and the 2Σg + state for S2 3+. The stability of the excited metastable states is expected, however, to be lower since they exhibit a lower barrier in comparison to the ground states of both ions. Their eventual experimental detection still appears to be possible.