The world's first kiloelectronvolt X-ray laser produces such a high flux of photons that atoms can be 'cored'. In other words, the light source can knock out both the electrons of an atom's innermost shell.

What would you do if someone gave you an X-ray source with a spectral brightness that was 10 billion times greater than anything that had previously existed? Most of us might work out that we could shine it on something and see what happens, but it requires considerable skill and insight to design and interpret an experiment that provides fundamental knowledge about how these ultra-intense X-rays interact with matter. Just such an experiment is reported by Young et al.1 on page 56 of this issue. The authors show how they can tailor the characteristics of the world's first X-ray laser of kiloelectronvolt (keV) photon energy in a way that enables them to 'peel' or 'core' atoms.

The X-ray source in question is the Linac Coherent Light Source at SLAC National Accelerator Laboratory in Menlo Park, California. This remarkable machine, which demonstrated lasing for the first time in April last year2 and started operating as a full user facility about six months later, promises a revolution in X-ray science. As is the case for all X-ray sources, it allows matter to be probed at the interatomic length scale, which is a few ångströms. What is transformative, however, is that it produces high-repetition-rate, ultra-bright pulses as short as a few tens of femtoseconds (1 femtosecond (fs) being 10−15 seconds). Such pulses are shorter than the typical vibrational period of atoms in a solid and thus allow, in principle, atomic motion to be charted in real time.

When the output of the X-ray source is focused to micrometre-scale spots, intensities in excess of 1018 watts per square centimetre are produced — a regime that until now was the province of optical lasers. At such high intensities, a single X-ray laser pulse can result in many X-ray photons being absorbed by an atom, a situation that is highly unlikely even when using the brightest sources previously available (those emitting synchrotron radiation). Understanding how light and matter interact in this new intensity regime is the goal of the team led by Young and colleagues1.

The authors focused the X-rays into a jet of neon gas by reflecting them at grazing angles of incidence from curved mirrors. They then detected the electrons and ions that were produced by accelerating them in a fixed electric field: electrons and ions of different charges travelled at different velocities and so could be differentiated by their time of arrival at the detector. Next, they varied the photon energy of the X-ray laser between 800 eV and 2,000 eV, as well as the length of the laser pulses (or, more precisely, the length of the electron bunches used to produce them) from 230 fs to 80 fs. By scanning these parameters and then analysing the charge states of the ions, they found that the X-rays could 'peel' or 'core' the atoms.

Neon atoms have ten electrons: two in the inner K shell; and eight in the outer L shell. It takes an 870-eV photon to knock out a K-shell electron. Thus, with photon energies lower than this value, the team1 observed peeling of the atoms; that is, the X-rays stripped off many of the eight weakly bound electrons from the L shell (Fig. 1a). With photon energies higher than 870 eV, they observed (as is seen with conventional X-ray sources) the ejection of one of the two core electrons, briefly leaving the ions in a state with a core hole and only one electron in the K shell. This occurs only briefly because the core hole is rapidly filled by an electron from the L shell, although at slightly higher photon energies another photon could then eject this electron as well, leading to a complex refilling and ionization process. However, before that refilling might occur, the remaining K-shell electron, which is no longer receiving the shielding that would be provided by its companion, becomes even more tightly bound to the neon nucleus. Indeed, a photon of energy 993 eV would be needed to eject this electron also and create two core holes. Figure 1: Peeling and coring of atoms. Young and colleagues1 have used an ultra-intense X-ray laser source to knock out electrons from neon atoms. a, Below an energy value of 870 eV — the energy it takes to knock out a K-shell electron — X-rays strip some of the eight weakly bound electrons from the outer L shell of the neon atom — a process that can be thought of as peeling the atom. b, Above 993 eV, both electrons from the inner K shell can be knocked out, ionizing the atom from the inside out — in other words, coring the atom. (Adapted from Encyclopædia Britannica.) Full size image

As Young et al. demonstrate, significant new physics occurs if the photon energy is higher than 993 eV. Beyond this critical energy, the photon density within the focal spot of the X-ray laser is so great that both of the inner electrons can be ejected during the laser pulse before refilling, thereby ionizing the atoms from the inside out — which can be termed coring of the atoms (Fig. 1b). When an atom has had both core electrons ejected, the main mechanism by which it absorbs X-rays is shut off until one of those core vacancies is filled by electrons from the L shell. This process usually takes a few femtoseconds, but the team1 found that this refilling time was itself extended at still higher photon energies of about 2,000 eV — an effect that they could observe with the shortest-duration laser pulses. The extension of the lifetime of the 'hollow' atom at high photon energies is attributed to an increase in the refilling time if some of the L-shell electrons are also ejected during the complex interaction (for example, if they fill the core hole but then themselves are ejected).

Questions remain, however. To obtain agreement between the data and theoretical modelling of the interaction between ultra-intense X-rays and atoms, the team needed to assume that the shortest X-ray pulses were considerably shorter than the length of the electron bunch that produced them — so perhaps the laser itself is not fully understood.

During the period in which an atom is cored, it is essentially transparent because the K-shell electrons are the ones that absorb X-rays most efficiently. At this stage in the 'evolution' of the ion, it becomes a better scatterer than absorber — what the team calls 'radiation hardening' of the atom. This phenomenon may have implications beyond fundamental atomic physics: one of the proposed flagship projects for X-ray lasers is single-biomolecule diffractive imaging, to determine the structures of proteins that cannot be crystallized3. This technique would work best with X-ray pulses that are shorter than the hollow-atom lifetime, enabling the diffraction patterns of proteins to be obtained before significant electron and nuclear motion takes place — an approach that has been dubbed 'diffract and destroy'. For this to be feasible, many electrons need to remain bound to their parent atoms long enough to scatter during the pulse. Although the jury is still deliberating on whether single-molecule imaging will become a reality, the work of Young and colleagues provides some degree of confidence in this approach.

References 1 Young, L. et al. Nature 466, 56–61 (2010). 2 http://home.slac.stanford.edu/pressreleases/2009/20090421.htm 3 Neutze, R., Wouts, R., van der Spoel, D., Weckert, E. & Hajdu, J. Nature 406, 752–757 (2000). Download references

Author information Affiliations Justin Wark is in the Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK. justin.wark@physics.ox.ac.uk Justin Wark Authors Justin Wark View author publications You can also search for this author in PubMed Google Scholar

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