Charge Fluctuations in Europium in Metallic Eu<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Si</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Recoilless-absorption measurements of the 21.6-keV $\ensuremath{\gamma}$ ray of ${\mathrm{Eu}}^{151}$ in Eu${\mathrm{Cu}}_{2}$${\mathrm{Si}}_{2}$ reveal an absorption line which has a strongly temperature-dependent isomer shift. The lines fall at neither the ${\mathrm{Eu}}^{3+}$ nor the ${\mathrm{Eu}}^{2+}$ valence positions, but in between. The results are interpreted in terms of fast fluctuations ($\ensuremath{\tau}&lt;3.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ sec) of an electron between the localized $4f$ level and the conduction band. The difference in energy between the ${\mathrm{Eu}}^{2+}$ and the ${\mathrm{Eu}}^{3+}$ states in Eu${\mathrm{Cu}}_{2}$${\mathrm{Si}}_{2}$ is found to be about 0.07 eV.