  We measured for the first time the electron density in an Extreme
Ultra-Violet induced plasma. This is achieved in a low-pressure argon plasma by
using a method called microwave cavity resonance spectroscopy. The measured
electron density just after the EUV pulse is $2.6\cdot10^{16}$ m$^{-3}$. This
is in good agreement with a theoretical prediction from photo ionization, which
yields a density of $4.5\cdot10^{16}$ m$^{-3}$. After the EUV pulse the density
slightly increase due to electron impact ionization. The plasma (i.e. electron
density) decays in tens of microseconds.
