Femtosecond electronic response of atoms to ultra-intense X-rays

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, ∼10 5 X-ray photons per Å 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. First strike from the LCLS The world's first X-ray free-electron laser — the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory in Menlo Park, California — came online last year. It opened a new era for studies at the atomic level, including the prospect of single-shot imaging of complex nano-objects such as biological molecules. The results of one of the first user experiments carried out at the LCLS are presented in this issue. The new facility produces ultrashort (femtosecond) pulses of high-intensity X-rays at a wavelength of less than 1.5 nm. The experiment examined the electronic response of free neon atoms to such radiation. During a single X-ray pulse, the atoms sequentially ejected all their ten electrons to produce fully stripped neon — 'hollow' atoms that are X-ray transparent. The authors explain the observations and underlying mechanisms of electron stripping using a straightforward model, which bodes well for further studies of interactions of the X-rays with more complex systems. With the start-up of the first X-ray free-electron laser, a new era has begun in dynamical studies of atoms. Here the facility is used to study the fundamental nature of the electronic response in free neon atoms. During a sin