Nature Photon. http://go.nature.com/qKGoRO (2012)

How many electrons can be kicked out of a xenon atom by an intense X-ray pulse? It seems the answer is more than theory would allow. Benedikt Rudek and colleagues have found unexpectedly highly charged xenon ions created in photo-ionization experiments using an X-ray free electron laser — the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory.

A short 1.5-keV X-ray flash can eject up to 36 electrons from a xenon atom — that is, two thirds of the total number. However, a sequential ionization process of one-photon absorption steps would only account for the ejection of 26 electrons, and at 2 keV an ionization limit of 32 electrons is supported by both theory and observations. From a comparison of experimental data with new calculations, Rudek et al. suggest that a previously unknown absorption mechanism — transient and resonance-enhanced — is responsible for the generation of the unusually highly charged ions, explaining why this occurs only at a certain energy.

Rudek and colleagues predict that transient resonance-enhanced absorption is in fact a general mechanism for the intense X-ray-induced ionization of heavy atoms — which could have implications for the creation of very highly charged dense plasmas and the diffractive imaging of samples containing heavy atoms.