  We report the observation of a novel nonlinear effect in the hard x-ray
range. Upon illuminating Fe and Cu metal foils with intense x-ray pulses tuned
near their respective K edges, photons at nearly twice the incoming photon
energy are emitted. The signal rises quadratically with the incoming intensity,
consistent with two-photon excitation. The spectrum of emitted high-energy
photons comprises multiple Raman lines that disperse with the incident photon
energy. Upon reaching the double K-shell ionization threshold, the signal
strength undergoes a marked rise. Above this threshold, the lines cease
dispersing, turning into orescence lines with energies much greater than
obtainable by single electron transitions, and additional Raman lines appear.
We attribute these processes to electron-correlation mediated multielectron
transitions involving double-core hole excitation and various two-electron
de-excitation processes to a final state involving one or more L and M
core-holes.
