What happens when an atom is excited by extremely strong X-ray photons such as an X-ray laser? A Stanford research group recently published a very exciting report on the ionization of neon (Z=10) by X-ray laser at the Linac Coherent Light Source (LCLS) housed at the SLAC National Accelerator Laboratory in California, USA. The laser used in this experiment is extremely powerful (1018 W/cm2, 105 X-ray photons/Å2), and the research group scanned the X-ray photon energy from 800 eV to 2,000 eV, as well as the pulse width from 80 fs to 230 fs. As the K absorption edge of neon is around 867 eV, below this energy, X-rays can strip some of the eight weakly bound electrons from the outer L shell of the neon atom. Such a process of peeling electrons from atoms would come as no surprise for readers of X-ray spectrometry. Above the absorption edge, K shell electrons are preferentially ejected, creating 1s vacancies that are refilled by electrons from the L shell. Before the relaxation occurs, the remaining K shell electron is even more tightly bound to the neon nucleus than in the ground state. Therefore, the K absorption edge for the system with a 1s vacancy is higher than usual. When the research team raised the X-ray photon energy to 993 eV, both electrons from the inner K shell were knocked out, ionizing the atom from the inside out - in other words, coring the atom. With this "hollow" neon then, a completely empty K shell has been created for the first time by X-ray photons, though similar phenomena may be possible by means of ultra-high temperature plasma, extremely high-energy collision processes etc. For more information, see the paper, "Femtosecond electronic response of atoms to ultra-intense X-rays", L. Young et al., Nature 466, 56 (2010). In the same issue, there is an interesting account by Justin Wark, "X-ray laser peels and cores atoms", Nature 466, 35 (2010).
