Multiphoton-induced X-ray emission at 4–5 keV from Xe atoms with multiple core vacancies

Abstract

SEVERAL recent experimental findings^1–3 have pointed to a possible route for making an X-ray laser, which could in principle provide an imaging system capable of molecular resolution⁴. The method involves the multiphoton excitation of atoms in van der Waals clusters or in molecules to yield ions with core-electron vacancies^1,2, which can then decay by emission of X-rays, in conjunction with a self-chanelling propagation mode of electromagnetic radiation³. The multiphoton excitation may be stimulated by ultrahigh-brightness, subpicosecond pulses of laser light⁵. We have previously observed² emission of X-rays from L-shell transitions in core-excited krypton atoms using this approach. Here we report the multiphoton production of X-rays of wavelength 2–3 Å from highly ionized xenon atoms which possess a large number of innershell vacancies while retaining several electrons in relatively weakly bound outer orbitals. Atoms with this ‘inverted’ electronic configuration are designated ‘hollow atoms’^6,7. We find that generation of hollow atoms can become the dominant excitation mode for such systems, making their exploitation in an X-ray laser a real possibility.