Physics Research Opportunities with XFEL’S

The advent of x-ray Free Electron Lasers will present a number of unique new scientific opportunities. These arise ti-om their special characteristics which include intensely brilliant pulses of x-rays delivered in very short times, complete transverse coherence, and high photon quantum degeneracy, amongst other things. While clearly much thought needs to be given to a detailed quantitative assessment of the feasibility of various experiments using such sources, including methods of dealing with heat loads on beamline optics and radiation damage to samples, there are a number of areas in which one can see new opportunities, and other exciting possibilities about which one might speculate. In this talk we shall briefly review some of these areas, such as x-ray holography, pump-probe type experiments, correlation spectroscopy and quantum optics experiments with x-rays. Successful construction and operation of a hard x-ray free electron laser (XFEL) source will open up many exciting new areas for scientific research, some of which we will attempt to survey in this article. We are still at an early stage in our collective thinking and so many of the ideas put forth here are admittedly speculative. Much more detailed examination of the feasibility of some of these ideas is in order (including realistic assessments of the radiation damage to the samples) and will undoubtedly be undertaken over the next few years, based on data available from the current thirdgeneration sources, detailed calculations of cross-sections, etc., and of course data from the Stanford LCLS when it comes on line. Many of these concepts have already been discussed in the proceedings of several workshops (l-5). We start by listing the factors that will make the XFEL unique for doing new science. These are: (a) peak brilliance (a factor of 1010-10’2over that at current sources), (b) short time pulses (-1 00 fs), which combined with (a) results in (c) extremely high electromagnetic fields for very short times, (d) extremely high quantum degeneracy of photon states (-1 0’2 times that obtainable at present at 1 ~) (e) completely coherent, diffraction limited beams, and (f) even a time-averaged brilliance which is at least four orders of magnitude higher than that at current sources, particularly at higher photon energies. Details of these order of magnitude increases are given in the paper by Cornaccia in these proceedings (6).