  The photoinduced valence dynamics of EuNi$_2$(Si$_{0.21}$Ge$_{0.79}$)$_2$ are
investigated using time-resolved X-ray absorption spectroscopy for Eu
$M_5$-edge. Through the pump-probe technique with synchrotron X-ray and
Ti:sapphire laser pulse, a photoinduced valence transition is observed from
Eu$^{3+}$ to Eu$^{2+}$. Because the lifetime of a photoinduced state can be up
to 3 ns, a metastable state is considered to be realized. By comparing the
experimental results with the theoretical calculations, the photoinduced
valence transition between Eu 4$f$ and conduction electrons is quantitatively
evaluated.
