The free electron laser x-ray source ARC-EN-CIEL will be by far more than a gradual improvement of today's storage ring based X-ray sources. It will be an entirely new kind of x-ray source with unprecedented performance in spectral, spatial and temporal resolution. Providing for the first time highly coherent X-ray pulses, the advent of ARC-EN-CIEL will stimulate equally revolutionary developments for X-ray science as the first laser have done in the optical regime. With ultrashort pulse durations on the femtosecond time scale and ultrahigh peak intensities comparable to the integral of 109 of currently available x-ray pulses, this source will enable experiments that we can only dream about today. And those envisioned experiments will only be the tip of the iceberg of what ARC-EN-CIEL will be known for – like in the case of any revolutionary development, the best applications of ARC-EN-CIEL we more than likely still have to discover.
Experiments envisioned to be performed at ARC-EN-CIEL originate from all areas of science pursued at currently available x-ray sources, demonstrating the breath of research activities ARC-EN-CIEL will render possible. This is also reflected in the variety of applications outlined in the following sections, which have been selected to emphasize the richness of scientific phenomena ARC-EN-CIEL will shine new light on. They address forefront scientific questions in atomic and molecular physics; cluster science; surface and interface science; magnetism, spintronics and nanophotonics; correlated materials; and biology. These examples are expected to change as our understanding of free electron laser based X-ray science will increases with the first sources becoming operational in the near future.
The scientific applications cover the study of time dependant phenomena on the scale of the femtosecond, the uses of the high power emitted by pulse, and the exploitation of the coherence of the emitted photons. The proposed energy range is perfectly adapted to reach the electronic, magnetic or chemical properties of materials.
Thus, in the field of magnetism (IPCMS, Strasbourg), ARC-EN-CIEL will make possible to approach the high-speed dynamics of magnetization (100 fs) in bringing the spin states selectivity which misses to the high-speed magneto-optics currently practised in the visible or the Infra-Red. The addition of the X microscopy techniques (L. Neel Institute, Grenoble, France) will enable to obtain very high space resolutions (< 20 nm, a major challenge for the miniaturization of the information storage devices.
By combining a laser excitation and the FEL, pump-probe type experiments will permit to approach the study of chemical reactivity on isolated systems (LIXAM) such as molecules adsorbed on the surface (LCPMR), or to study very fundamental processes of atomic and molecular physics in nonlinear mode (LIXAM).
Thus, it should allow to probe with "real" conditions (high pressure, liquids, clusters and nanoparticles etc...) catalitic interest chemical reactions (LCPMR, Univ. Uppsala). By applying techniques of X-rays diffraction or X-rays absorption resolved in time, we plan to determine the molecular structure of a transitory state and its dynamics, and to study the chemical environments or the electronic states involved during the reaction process (GCM, Rennes, France).
It is clear, in the field of imagery resolved in time, that a sources as ARC-EN-CIEL will open completely new prospectives to carry out experiments with a completely new approach. Indeed, the coherence of FEL radiation could benefit to femtosecond time scale imaging by Fourier transform holography (LCPMR), by using only one irradiation flash on a single sample. This type of experiment, without needing average on several pulses, opens the way towards an immense investigation field.
We could, for example, measure by imaging the fluctuations of spin block in balance to the proximity of a magnetic phase transition. The application to biological objects (cells) would be also very promising.
