Nonlinear atomic response to intense, ultrashort x rays

The nonlinear absorption mechanisms of neon atoms to intense, femtosecond \textit{kilovolt} x rays are investigated. The production of Ne$^{9+}$ is observed at x-ray frequencies below the Ne$^{8+}$, 1$s^2$ absorption edge and demonstrates a clear quadratic dependence on fluence. Theoretical analysis shows that the production is a combination of the 2-photon ionization of Ne$^{8+}$ ground state and a high-order sequential process involving single-photon production and ionization of transient excited states on a time-scale faster than the Auger decay. We find that the nonlinear direct two-photon ionization cross-section is orders of magnitude higher than expected from previous calculations.