Photoelectron spectroscopic study of the E ⊗ e Jahn-Teller effect in the presence of a tunable spin-orbit interaction. III. Two-state excitonic model accounting for observed trends in the $\tilde{\rm X}^{\,2}{\rm {E}}$X̃2E ground state of $\rm {CH}_3\rm {

Open-shell molecules in doubly degenerate 2E electronic states are subject to the E ⊗ e Jahn-Teller effect and spin-orbit interactions. The rotational structure of the ground vibrational level of the \documentclass[12pt]{minimal}\begin{document}$\tilde{\rm X}^+$\end{document}X̃+ 2E ground state of CH3F+ has been observed by high-resolution photoelectron spectroscopy. In contrast to what is observed in other members of the isoelectronic families \documentclass[12pt]{minimal}\begin{document}$\rm {CH}_3\rm {X}^+\,(\rm {X}=\rm {Cl,\,Br,\,I})$\end{document} CH 3X+(X= Cl , Br ,I) and \documentclass[12pt]{minimal}\begin{document}$\rm {CH}_3\rm {Y}\,(\rm {Y}=\rm {O,\,S})$\end{document} CH 3Y(Y=O,S), the spin-orbit interaction does not lead to a splitting of the ground state of CH3F+. Observed trends in the spectra of the \documentclass[12pt]{minimal}\begin{document}$\tilde{\rm X}$\end{document}X̃ 2E ground states of these molecules are summarized. Whereas certain trends, such as the reduction of the observable effects of the Jahn-Teller interactions and the increase of the spin-orbit splitting with increasing nuclear charge of X and Y are easily understood, other trends are more difficult to explain, such as the much reduced spin-orbit splitting in \documentclass[12pt]{minimal}\begin{document}$\rm {CH}_3\rm {F}^+$\end{document} CH 3F+ compared to \documentclass[12pt]{minimal}\begin{document}$\rm {CH}_3\rm {O}$\end{document} CH 3O. A simple two-state excitonic model is used to account for the trends observed within the series of the methyl-halide radical cations and also the similarities and differences between \documentclass[12pt]{minimal}\begin{document}$\rm {CH}_3\rm {F}^+$\end{document} CH 3F+ and the isoelectronic \documentclass[12pt]{minimal}\begin{document}$\rm {CH}_3\rm {O}$\end{document} CH 3O radical. Within this model, the electron hole in the 2E ground states of \documentclass[12pt]{minimal}\begin{document}$\rm {CH}_3\rm {X}^+$\end{document} CH 3X+ and \documentclass[12pt]{minimal}\begin{document}$\rm {CH}_3\rm {Y}$\end{document} CH 3Y is described in terms of contributions from the halogenic (or chalcogenic) px, y orbitals and the pyramidal-methylic (e) orbitals. This model enables a global, semi-quantitative description of the combined effects of the Jahn-Teller and spin-orbit interactions in these molecules and also a simple interpretation of the spin-orbit-coupling reduction factor ζe.