Abstract
A calculation model is derived for taking into account quadratic, in addition to linear, coupling Jahn—Teller effects in determining vibronic energy levels and transitions. Procedures are developed for analysis of Jahn—Teller electronic spectra on this basis and the new features, with respect to the linear coupling approximation, brought about by introduction of quadratic coupling, are discussed. Vibronic analyses of the B̃ 2 A″ 2-X̃ 2E″ transitions of 1,3,5-C 6F 3H + 3, 1,3,5-C 6F 3D + 3 and 1,3,5-C 6Cl 3H + 3 are carried out, in particular for bands involving excitation of the mode 6 vibration. Experimental evidence for quadratic Jahn—Teller effects is obtained for the sym-trifluorobenzene ions and the linear coupling parameters D 6, ω 6 and quadratic coupling parameter q 6 are derived. Two possible orders of magnitude of the quadratic coupling strength are found to be compatible with the spectra of 1,3,5-C 6Cl 3H + 3. The analyses are consistent between the three ions and are not in contradiction with the general findings based on the linear approximation alone.
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