An era of exploring the interactions of high-intensity, hard X-rays with matter has begun with the start-up of a hard-X-ray free-electron laser, the Linac Coherent Light Source (LCLS). Understanding how electrons in matter respond to ultra-intense X-ray radiation is essential for all applications. Here we reveal the nature of the electronic response in a free atom to unprecedented high-intensity, short-wavelength, high-fluence radiation (respectively 10(18) W cm(-2), 1.5-0.6 nm, approximately 10(5) X-ray photons per A(2)). At this fluence, the neon target inevitably changes during the course of a single femtosecond-duration X-ray pulse-by sequentially ejecting electrons-to produce fully-stripped neon through absorption of six photons. Rapid photoejection of inner-shell electrons produces 'hollow' atoms and an intensity-induced X-ray transparency. Such transparency, due to the presence of inner-shell vacancies, can be induced in all atomic, molecular and condensed matter systems at high intensity. Quantitative comparison with theory allows us to extract LCLS fluence and pulse duration. Our successful modelling of X-ray/atom interactions using a straightforward rate equation approach augurs favourably for extension to complex systems.

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We have measured absolute triple differential cross sections for photo-double ionization of helium at 20 eV excess. The measurement covers the full ranges of energy sharing and emission angles of the two photoelectrons. We compare our data for selected geometries with the convergent close-coupling (CCC) calculations as well as 2SC calculations by Pont and Shakeshaft and 3C calculations by Maulbetsch and Briggs. In terms of the absolute magnitude and the trend in the shapes of the triple differential cross section for different geometries we find good agreement of the CCC and published 2SC calculations with our measurement, though differences with respect to the observed shape of individual patterns still exist.

We show that high fluence, high-intensity x-ray pulses from the world's first hard x-ray free-electron laser produce nonlinear phenomena that differ dramatically from the linear x-ray-matter interaction processes that are encountered at synchrotron x-ray sources. We use intense x-ray pulses of sub-10-fs duration to first reveal and subsequently drive the 1s↔2p resonance in singly ionized neon. This photon-driven cycling of an inner-shell electron modifies the Auger decay process, as evidenced by line shape modification. Our work demonstrates the propensity of high-fluence, femtosecond x-ray pulses to alter the target within a single pulse, i.e., to unveil hidden resonances, by cracking open inner shells energetically inaccessible via single-photon absorption, and to consequently trigger damaging electron cascades at unexpectedly low photon energies.

The triple differential cross section for double photoionization in helium has been measured for the first time. Equal energy sharing and a simple geometry for the two emitted electrons were chosen in order to facilitate the comparison with theory. Good agreement between experimental and theoretical data is found for the angular correlation pattern if the mutual Coulomb repulsion between both escaping electrons is taken into account.PACS numbers: 32.80.Fb Double photoionization-it might be in one of the two clearly defined limiting forms of a direct process with the simultaneous emission of two photoelectrons or a two-step process with the sequential emission of a photoelectron and an Auger electron, or even a mechanism in between -is due to electron correlations. Helium is the simplest correlated system that shows only direct double ionization and this for all photon energies above the double ionization threshold. Hence, it provides the ideal test case for theoretical treatments of three-particle breakup occurring in double photoionization, if the angle-and energyresolved triple differential cross section d 3 a/dft\dQ,2dE (TDCS) is considered. In spite of this importance only three theoretical predictions exist for the TDCS in helium [1][2][3], and experiments on energy-and angle-resolved photon-induced two-electron emission have hitherto concentrated on other systems (double photoionization in the outer p shell of krypton [4,5], xenon [5,6], and argon [7]; two-step double photoionization in xenon [8]). In this Letter we report the first experimental results for the TDCS of direct double photoionization in helium and compare them with theory.The experiment has been performed at the electron storage ring BESSY in Berlin at the toroidal grating monochromator TGM4 by applying the method of angle-resolved electron spectrometry and measuring the two ejected electrons in coincidence in two separate spectrometers. The rate of true coincidences was of the order of 20 MHz; accidental coincidences were recorded simultaneously with the true ones and the ratio of true to accidental coincidences typically varied between 1 and 0.2, depending on the actual parameters.The following parameters which determine the angular correlation pattern have been selected: (i) A photon energy of 99.0 eV (bandwidth 0.35 eV) leads to an excess energy Zsexc^lO.O eV which provides kinetic energies for the ejected electrons large enough to avoid possible disturbances in spectrometer transmission ("cutoff" effect at low kinetic energies) or by scattered electrons, (ii) The pass energies of the electron spectrometers are set to transmit only electrons which share the available excess energy equally (E \ =£ , 2 ==