X-ray magnetic-circular-dichroism spectra on the superparamagnetic transition-metal ion clusters Mn12 and Fe8

X-ray magnetic-circular-dichroism (XMCD) spectra on the molecular superparamagnets $[{Mn}₁₂{O}₁₂{(CH}₃{COO)}₁₆{(H}₂{O)}₂₄]{·}{2CH}₃{COOH{{·}}4H}₂O$ (in short Mn12) and $[{Fe}₈{O}₂{(OH)}₁₂{(tacn)}₆{]Br}₂{{{·}}9H}₂O$ (in short Fe8) are presented. The XMCD measurements have been performed at the ${Mn{-}L}III,II$ edges in the Mn12 compound and at the ${Fe{-}L}III,II$ edges in the Fe8 compound. For Fe8 the typical two-peak structure of Fe(III) compounds is found. For Mn12 and Fe8, from the dichroic signals a very low or negligible $〈{L}z〉$ was obtained: to the best of our knowledge, this is the first direct experimental evidence of the quenching of the angular momentum by the crystal field in these systems. Quantitative analysis of the dichroic spectra for Fe8 yielded 〈Lz〉/〈Sz〉=0.02(1).