Short x-ray pulses from free electron lasers (FEL) open a new regime for investigating multi-photon processes. The first x-ray FEL, the Linac Coherent Light Source (LCLS), is in operation, providing intense short pulses for investigating the creation and decay of multiple core-holes.
The response of molecular systems to the ultra-intense, femtosecond x-ray radiation was investigated. Sequential multiphoton ionization, frustrated absorption  and double core hole production mechanisms [2,3] will be presented.
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 N. Berrah, Proc. Natl. Acad. Sci. USA (PNAS), 108, issue 41, 16912 (2011).