| Summary: | A model has been developed to describe the effects of the Jahn-Teller (JT) effect on the magnetic circular dichroism (MCD) and absorption spectra of axial molecules. It treats spin-orbit and Jahn-Teller coupling on equal footing and, in terms of the latter, allows for both first- (linear) and second-order (quadratic) coupling in both ground arid excited states.
Three axial radicals, trapped in solid Ar matrices, were chosen to test the utility of the model The ferricenium radical cation, Fe(cp)2+, and the cylcopentadienyl radical, C5H5., were generated by photoionisation/photolysis involving a microwave discharge in Ar, then trapped in solid Ar. Their MCD and absorption spectra were then collected on a simultaneous MCD/double-beam absorption spectrometer, MOD4, between temperatures 1.6 and 13 K and magnetic fields of 0 - 5T. The MCD and absorption spectra for the third matrix-isolated radical, the cycloactatetrene radical anion in Ar, COT/Ar, were taken from the literature.
The data collected for C5H5./Ar were insufficient to allow a useful analysis. The spectra of, Fe(cp)2+ /Ar and COT-/Ar, on the other hand, show extensive vibrational structure. Quantitative discussion is presented for these two species. JT effects are identified and characterised in the ground (2E2g) and excited (2E1U) states of Fe(cp)2+, and in the excited (2E1g) state of COT-. Herzberg-Teller coupling is found to play an important role in the 2E1g← 2E2u transition of COT-.
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