Exploring the Jahn-Teller and pseudo-Jahn-Teller conical intersections in the ethane radical cation

Abstract

We report a theoretical account on the static and dynamic aspects of the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) interactions in the ground and first excited electronic states of the ethane radical cation. The findings are compared with the experimental photoionization spectrum of ethane. The present theoretical approach is based on a model diabatic Hamiltonian and with the parameters derived from ab initio calculations. The optimized geometry of ethane in its electronic ground state (¹A_1g) revealed an equilibrium staggered conformation belonging to the D_3d symmetry point group. At the vertical configuration, the ethane radical cation belongs to this symmetry point group. The ground and low-lying electronic states of this radical cation are of ²E_g, ²A_1g, ²E_u, and ²A_2u symmetries. Elementary symmetry selection rule suggests that the degenerate electronic states of the radical cation are prone to the JT distortion when perturbed along the degenerate vibrational modes of eg symmetry. The ²A_1g state is estimated to be ∼0.345eV above the ²E_g state and ∼2.405eV below the ²E_u state at the vertical configuration. The symmetry selection rule also suggests PJT crossings of the ²A_1g and the ²E_g electronic states of the radical cation along the vibrational modes of e_g symmetry and such crossings appear to be energetically favorable also. The irregular vibrational progressions, with numerous shoulders and small peaks, observed below 12.55 eV in the experimental recording are manifestations of the dynamic (E⊗e)-JT effect. Our findings revealed that the PJT activity of the degenerate vibrational modes is particularly strong in the ²E_g–²A_1g electronic manifold which leads to a broad and diffuse structure of the observed photoelectron band.