Non-linear processes in the interaction of atoms and molecules with intense EUV and X-ray fields from SASE free electron lasers (FELs)

The advent of free electron laser (FEL) facilities capable of delivering high intensity pulses in the extreme-UV to X-ray spectral range has opened up a wide vista of opportunities to study and control light matter interactions in hitherto unexplored parameter regimes. In particular, current short wavelength FELs can uniquely drive non-linear processes mediated by inner shell electrons and in fields where the photon energy can be as high as 10 keV and so the corresponding optical period reaches below one attosecond. Combined with ultrafast optical lasers, or simply employing wavefront division, pump probe experiments can be performed with femtosecond time resolution. As single photon ionization of atoms and molecules is by now very well understood, they provide the ideal targets for early experiments by which not only FELs can be characterised and benchmarked but can also be the natural departure point in the hunt for non-linear behaviour of atomistic systems bathed in laser fields of ultrahigh photon energy. In this topical review we illustrate with specific examples the gamut of apposite experiments in atomic, molecular physics currently underway at the SCSS Test Accelerator (Japan), FLASH (Hamburg) and LCLS (Stanford).