Multistate and multimode vibronic dynamics: The Jahn-Teller and pseudo-Jahn-Teller effects in the ethane radical cation

Multimode Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) coupling effects in the photoelectron spectrum of ethane are theoretically investigated. In this article, we focus on the vibronic structure of the second excited (B) over tilde E-2(u) electronic manifold of the ethane radical cation which reveals an asymmetric band in the 14.5-16.5 eV ionization energy range in the experimental recordings. Ionization of an electron from the third occupied le(u) molecular orbital of ethane produces the radical cation in the degenerate (B) over tilde E-2(u) electronic manifold, which is prone to the JT instability when distorted along the degenerate e. vibrational modes. The theoretical formalism employed here is based on a model diabatic Hamiltonian and a quadratic vibronic coupling scheme with the parameters derived from ab initio electronic structure calculations. The photoelectron band is calculated by carrying out quantum dynamical simulations in the coupled manifold of electronic states. The (B) over tilde E-2(u) electronic manifold of the radical cation is estimated to be similar to 2.75 eV and similar to 2.40 eV above its (X) over tilde E-2(g) and the (A) over tilde (2)A(lg) electronic states, respectively. The symmetry selection rule suggests PJT coupling of these electronic states along the vibrational modes of e(g)/e(u) symmetry. The quadratic JT spectrum simulated within the (B) over tilde E-2(u) electronic manifold shows two maxima at similar to 14.96 eV and similar to 15.76 eV which are attributed to the two JT split adiabatic sheets of this electronic manifold. This is in good accord with their position observed at similar to 15.0 eV and similar to 15.8 eV, respectively, in the experimental recording. The diffuse structure of the overall band can be accounted to a large extent by considering the (A) over tilde (2)A(lg) - (B) over tilde E-2(u) PJT interactions. The overall shape of the theoretical band agrees very well with the experimental results. Further refinement of the theoretical results may be accomplished by including (X) over tilde E-2(g) - (A) over tilde (2)A(lg) - (B) over tilde E-2(u) PJT interactions in the theoretical model. Importance of the latter vibronic interactions is also discussed in the text. (c) 2006 Elsevier B.V. All rights reserved.