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 [1] and double core hole production mechanisms [2,3] will be presented.

[1] M. Hoener et al., Phys. Rev. Lett. 104, 253002  (2010)

[2] L. Fang et al., Phys. Rev. Lett 105, 083005 (2010).

[3] N. Berrah, Proc. Natl. Acad. Sci. USA (PNAS), 108, issue 41, 16912 (2011).