Exchange interactions in polynuclear complexes

We present detailed optical and magneto-optical data on Cs₃Cr₂Cl₉ as a single crystal containing only pairs of Cr³⁺ ions. Measurements have been carried out in a wide range of temperatures down to pumped helium and in the whole spectral region covering all singly and several doubly excited states. MCD helped considerably in resolving the absorption spectra into their numerous components, and proved especially useful in demonstrating the occurrence of a double excitation to ⁴ T ₂. It provides also parallel g factors for several excited terms.

The main conclusions of our work are: (i) all transitions are electric dipole allowed; (ii) transitions to singly excited doublet states are essentially promoted via a single ion mechanism; (iii) vibronic interaction must be invoked to understand the absorption data on sharp lines; (iv) the trigonal plus spin-orbit interaction competes favourably with exchange interaction to produce excited states splittings.

We present also a new non-phenomenological model to describe the splitting of the ⁴ A ₂Θ² E manifold under the combined effects of exchange, trigonal and spin-orbit interaction. This model includes no adjustable parameter. It leads to λ₀ ～ 180 cm^-1 and offers a good starting basis for a coherent interpretation of most of our experimental data.