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 $\rm {CH}_3\rm {X}^+\,(\rm {X}=\rm {F,\,Cl,\,Br,\,I})$ CH 3X+(X=F, Cl , Br ,I) and $\rm {CH}_3\rm {Y}\,(\rm {Y}=\rm {O,\,S})$ CH 3Y(Y=O,S)

Abstract

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.

Rotational and fine structure of open-shell molecules in nearly degenerate electronic states

“…One may take the SO splitting of the OH radical 43 (-139 cm -1 ) as the estimated value for a . 5,42 Coupled-cluster calculations predict that e aζ is 131 cm -1 for both conformers of 1propoxy. 75 e ζ is therefore estimated to be 0.942, and so is t ζ based on the discussion above.…”

“…In spectroscopic analysis, it is often advantageous to partition the effective SO constant into different factors (see Section 7) ceases to be a good quantum number, and the SO interaction is quenched not only electronically 41,42 but also vibronically 3,[19][20] through mixing of vibronic basis functions. For instance, the groundstate SO splitting of the OH( 2…”

Adiabatic ionization energies of the overlapped A2A1and B2E electronic states in CH3Cl+/CH3F+measured with double imaging electron/ion coincidence

“…3,4,14 The techniques of threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion coincidence (TPEPICO) have been frequently employed to measure the values of ionization energy and fragment appearance energy due to the high collection efficiency of threshold electrons associated with a high and constant electron energy resolution. [22][23][24][25][26][27][28][29][30][31][32][33][34] The valence shell molecular configurations of CH 3 Cl and CH 3 F in their X 1 A 1 electronic ground state are known as (1a 1 ) 2 (2a 1 ) 2 (1e) 4 (3a 1 ) 2 (2e) 4 , where their core shells have been neglected for simplicity. Indeed, in photoionization processes the molecular ions may be prepared in different states depending on the photon energy and transition possibility.…”

Spin–orbit coupling and rovibrational structure in the iododiacetylene radical cation by PFI-ZEKE photoelectron spectroscopy

“…Rather than adapting the model parameters to get the best possible agreement with the level positions measured in this work and in previous studies, we fixed all parameters to experimentally measured ionisation energies and to interactions derived from the spectra of other molecules. We used (i) a value of 84886 cm −1 for the ionisation energy E I,X /(hc) of the iodine atom, which corresponds to the difference between the centres of gravity of the three spin-orbit components of the 3 P ground state of I + and the two spin-orbit components of the 2 P ground state of I [7], (ii) a value of 91953.5 cm −1 for the ionisation energy E I,HC 2 H /(hc) of acetylene [38], (iii) a value of 7632 cm −1 for the charge-transfer coupling parameter V I-CC derived from the photoelectron spectrum of HC 2 I [8], and (iv) a value of 9840 cm −1 for the charge-transfer coupling parameter V ππ extracted from the photoelectron spectrum of diacetylene [20,21], as detailed in Table 2.…”

“…On the other hand, the p π -orbitals of the halogen substituent can contribute to the conjugated π-electron system of the chain, thus strengthening the resonance interaction within the molecule [1]. Photoelectron spectroscopy is well suited to study the charge distribution in 2 Π electronic states of positively charged carbon chains with halogen end groups because the observed spin-orbit splittings are directly proportional to the p π electron-hole density at the halogen atom [2][3][4][5][6][7][8].…”