Ralph Schenker, Myriam Triest, Christian Reber and Hans U. Güdel
Interference Dips in the Single-Crystal Absorption Spectrum of the Tri-m-fluoro-Bis(trifluorochromate(III)) Complex
Inorg. Chem. 40, 5787-5794 (2001)                       

Abstract: The lowest energy spin-allowed crystal field band of (Et₄N)₃Cr₂F₉ shows a distinct interference dip at 15000 cm^–1 with an approximate width of 200 cm^–1. This spectroscopic feature is due to spin-orbit coupling between the ²E and ⁴T₂ excited states and is analyzed with a set of two coupled potential energy surfaces. The minimum of the ⁴T₂ potential surface is displaced along at least two normal coordinates. The modes involved cannot be directly determined from the unresolved absorption spectrum, but are obtained from Raman spectra and from the well-resolved spin-forbidden crystal field transition to the ²A₁ state. The first mode with a frequency of 415 cm^–1 has predominant Cr-F stretching character; the second mode has a frequency of 90 cm^–1 and involves the entire Cr₂F₉^3- dimer.