Resonance Raman spectrum of the tetrabromoferrate(III) ion

Abstract

A resonance Raman spectrum has been obtained for the complex [(C₂H₅)₄N][FeBr₄] in the solid state. At least two progressions in the ν₁(a₁) fundamental have been observed, viz.nν₁(to n= 7) and ν₃+nν₁(to n= 5) and/or ν₄+nν₁(to n= 6). The spectroscopic constants ω₁and x₁₁ have been determined to be 202.0 and –0.38 cm_–1 respectively from an analysis of the nν₁ progression. Excitation profiles have been measured for ν₁ and ν₃(Stokes and anti-Stokes) and for 2ν₁(Stokes); these profiles demonstrate the different resonance conditions for Stokes and anti-Stokes Raman bands. A more detailed excitation profile has been measured for the ν₁(Stokes) fundamental in the region of one of the ligand field transitions. This profile has a minimum at approximately ν₀₀ and a maximum shifted by 250 ± 50 cm^–1 towards lower wavenumbers, an observation which is attributed to interference effects. The ν₁ fundamental has been found to exhibit unusual (so called “anomalous”) polarisation at resonance.