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.

T heory predicts 1-4 that, with an ultrashort and extremely bright coherent X-ray pulse, a single diffraction pattern may be recorded from a large macromolecule, a virus or a cell before the sample explodes and turns into a plasma. Here we report the first experimental demonstration of this principle using the FLASH soft-X-ray free-electron laser. An intense 25 fs, 4 × 10 13 W cm −2 pulse, containing 10 12 photons at 32 nm wavelength, produced a coherent diffraction pattern from a nanostructured non-periodic object, before destroying it at 60,000 K. A novel X-ray camera assured single-photon detection sensitivity by filtering out parasitic scattering and plasma radiation. The reconstructed image, obtained directly from the coherent pattern by phase retrieval through oversampling 5-9 , shows no measurable damage, and is reconstructed at the diffraction-limited resolution. A three-dimensional data set may be assembled from such images when copies of a reproducible sample are exposed to the beam one by one 10 .X-ray free-electron lasers (FELs) are expected to permit diffractive imaging at high resolutions of nanometre-to micrometre-sized objects without the need for crystalline periodicity in the sample [1][2][3][4] . Structural studies within this size domain are particularly important in materials science, biology and medicine. Radiation-induced damage and sample movement prevent the accumulation of high-resolution scattering signals for such samples in conventional experiments 11,12 . Damage is caused by energy deposited into the sample by the very probes used for imaging, for example photons, electrons or neutrons. At X-ray frequencies, inner-shell processes dominate the ionization of the sample; photoemission is followed by Auger or fluorescence emission and shake excitations. The energies of the ejected photoelectrons, Auger electrons and shake electrons differ from each other, and these electrons are released at different times, but within about ten femtoseconds, following photoabsorption 1,13 . Thermalization of the ejected electrons through collisional electron cascades is completed within 10-100 fs (refs 14,15). Heat transport, diffusion and radical reactions take place over some picoseconds to milliseconds.The effect of X-ray-induced sample damage on the recorded image or diffraction pattern could be substantially reduced, if we could collect diffraction data faster than the relevant damage processes 1,16 . This approach requires very short and very bright X-ray pulses, such as those expected from a short-wavelength FEL. However, the large amount of energy deposited into the sample by a focused FEL pulse will ultimately turn the sample into a plasma. The question is when exactly would this happen. There are no experiments with X-rays in the relevant time and intensity nature physics VOL 2 DECEMBER 2006 www.nature.com/naturephysics

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Sequential multiple photoionization of the prototypical molecule N 2 is studied with femtosecond time resolution using the Linac Coherent Light Source (LCLS). A detailed picture of intense x-ray induced ionization and dissociation dynamics is revealed, including a molecular mechanism of frustrated absorption that suppresses the formation of high charge states at short pulse durations. The inverse scaling of the average target charge state with x-ray peak brightness has possible implications for singlepulse imaging applications.

Abstract:The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features were resolved in time-dependent x-rayinduced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. The largest contribution to the jitter noise spectrum was the locking of the laser oscillator to the reference RF of the accelerator, which suggests that simple technical improvements could reduce the jitter to better than 50 fs. ©2010 Optical Society of America

The interaction of intense extreme ultraviolet femtosecond laser pulses ( 32:8 nm) from the FLASH free electron laser (FEL) with clusters has been investigated by means of photoelectron spectroscopy and modeled by Monte Carlo simulations. For laser intensities up to 5 10 13 W=cm 2 , we find that the cluster ionization process is a sequence of direct electron emission events in a developing Coulomb field. A nanoplasma is formed only at the highest investigated power densities where ionization is frustrated due to the deep cluster potential. In contrast with earlier studies in the IR and vacuum ultraviolet spectral regime, we find no evidence for electron emission from plasma heating processes.

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.

X-ray free-electron lasers (FELs) produce femtosecond x-ray pulses with unprecedented intensities that are uniquely suited for studying many phenomena in atomic, molecular, and optical (AMO) physics. A compilation of the current developments at the Linac Coherent Light Source (LCLS) and future plans for the LCLS-II and Next Generation Light Source (NGLS) are outlined. The AMO instrumentation at LCLS and its performance parameters are summarized. A few selected experiments representing the rapidly developing field of ultra-fast and peak intensity x-ray AMO sciences are discussed. These examples include fundamental aspects of intense x-ray interaction with atoms, nonlinear atomic physics in the x-ray regime, double core-hole spectroscopy, quantum control experiments with FELs and ultra-fast x-ray induced dynamics in clusters. These experiments illustrate the fundamental aspects of the interaction of intense short pulses of x-rays with atoms, molecules and clusters that are probed by electron and ion spectroscopies as well as ultra-fast x-ray scattering.