Optical absorption spectra of Fe2+ and Fe3+ in aqueous solutions and hydrated crystals

Absorption spectra of aqueous solutions containing Fe2+ and Fe3+ ions and hydrated crystals are studied up to 50000 cm–1. Charge‐transfer transitions for both hydrated ions are here measured for the first time. The weak Fe2+ spin‐forbidden transitions which up to now have received little attention in diluted systems, are put in evidence: as a consequence, we have proposed for the lower energy transitions different attributions respect to the commonly accepted ones. It is further shown that the step by step oxidation process of Fe2+ to Fe3+, starting from Fe(BF4)2 solution, allows us to quantitatively correlate the oscillator strength of the various spin‐allowed, spin‐forbidden and charge‐transfer transitions of both ions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)