Pair spectra studies of Cr3+ions in the (4A2×2T1) excited states of Cs3Cr2Cl9

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Abstract
A detailed optical absorption and Zeeman study of the spectrum of exchange coupled Cr3+ ions in Cs3Cr2Cl9 is presented, with particular reference to the singly-excited state where one ion of the pair is in the 4A2 ground state and the other is in the 2T1 optically excited state. The results are explained in terms of a new theoretical approach (K.R. Barry et al., ibid., vol.14, p.1281, 1981). Whereas the gross features of the spectrum may be understood without consideration of spin-dependent effects, it is shown that a small second-order spin-orbit interaction is needed to explain the fine details of the spectrum. In addition, an analysis of the vibronic structure associated with the pair transitions shows that the dominant spin-phonon coupling is to A1g modes which modulate the separation of the Cr3+ pair.