Multimode Jahn–Teller and pseudo-Jahn–Teller coupling effects in the photoelectron spectrum of CH₃F

Abstract

The multimode Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) coupling effects in the photoelectron spectrum of methylfluoride are theoretically investigated with the aid of an ab initio quantum dynamical approach. The theoretical findings are compared with the experimental results of Karlsson et al. [Phys. Scripta 16 (1977) 225]. At the vertical configuration, the ground, first and second excited electronic states of the methylfluoride radical cation belong to the ²E, ²A₁ and ²E symmetry species. The three doubly degenerate vibrational modes cause a splitting of the degeneracy of the E electronic states of the radical cation and exhibit the (E⊗e)-JT effect. The same vibrational modes may also cause a coupling of the degenerate and the non-degenerate electronic states (PJT effect). In our theoretical approach we devise a model diabatic vibronic Hamiltonian within a quadratic vibronic coupling scheme. The parameters of the Hamiltonian are derived by performing extensive ab initio calculations at the CASSCF-MR-AQCC level of theory. The photoelectron bands are calculated by a quantum dynamical approach based on the Lanczos algorithm. A detailed examination of the various static and dynamic aspects of the problem reveals an interesting interplay of JT and PJT coupling effects in the photoelectron-induced dynamics of CH₃F⁺. The resolved progression in the first photoelectron band corresponding to the ground X˜²E electronic manifold of CH₃F⁺ is found to be mainly caused by the C–F stretching (a1), C–F bending (e) and CH₃ deformation (e) modes. The JT coupling effects are not particularly strong in this electronic manifold. This also holds for the second photoelectron band attributed to the vibronic structure of the òA1 and B˜²E electronic states of CH₃F⁺. However, the extremely large Condon activity of the symmetric vibrational modes in the òA1 electronic state contributes much to the broad and highly diffuse nature of this band which is also observed experimentally. The PJT activity of the anti-symmetric C–F bending (e) vibration in the X˜²E and B˜²E electronic states is also discussed.