  Here, we report on the nonlinear ionization of argon atoms in the short
wavelength regime using ultraintense x rays from the European XFEL. After
sequential multiphoton ionization, high charge states are obtained. For photon
energies that are insufficient to directly ionize a $1s$ electron, a different
mechanism is required to obtain ionization to Ar$^{17+}$. We propose this
occurs through a two-color process where the second harmonic of the FEL pulse
resonantly excites the system via a $1s \rightarrow 2p$ transition followed by
ionization by the fundamental FEL pulse, which is a type of x-ray
resonance-enhanced multiphoton ionization (REMPI). This resonant phenomenon
occurs not only for Ar$^{16+}$, but through multiple lower charge states, where
multiple ionization competes with decay lifetimes, making x-ray REMPI
distinctive from conventional REMPI. With the aid of state-of-the-art
theoretical calculations, we explain the effects of x-ray REMPI on the relevant
ion yields and spectral profile.
