After having reached the status of user facilities in the infrared, Free Electron Laser (FEL) oscillators are becoming interesting light sources for scientific research also in the UV/VUV wavelength region. Thanks to continuous advances in accelerator and mirror technology, it has been possible to reach with tunability energies which are hardly accessible with conventional lasers (the European FEL project at Elettra recently lased below 190 nm), with the realistic perspective of reaching even shorter wavelengths in the near future. Thanks to their unique properties, such as tunability, full coherence, high intensity, spectral and temporal stability, they offer new opportunities for many kinds of spectroscopy: in particular, storage ring FELs are ideal sources for pump–probe experiments in conjunction with synchrotron radiation, as demonstrated by a series of studies of the nonequilibrium space charge distribution at photoexcited Si surfaces and interfaces performed at SuperACO. Besides, especially when operated at shorter wavelengths, FEL oscillators are very attractive also for one-photon experiments requiring high power and full tunability. We describe here the properties of the sources, and provide an overview of the experiments that have been performed and that are planned to exploit the new opportunities they offer.
Time-resolved free-electron laser spectroscopy of a copper isoelectronic center in silicon