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

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 E-2, (2)A(1) and E-2 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 circle times 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) over tilde E-2 electronic manifold of CH3F+ is found to be mainly caused by the C-F stretching (al), 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 (A) over tilde (2)A(1) and E-2 electronic states of CH3F+. However, the extremely large Condon activity of the symmetric vibrational modes in the (A) over tilde (2)A(1) 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) over tilde E-2 and (B) over tilde E-2 electronic states is also discussed. (C) 2004 Elsevier B.V. All rights reserved.