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Conceptual Design Report

CDR chapter 3 - The Scientific Case

Chemical, physical and biological processes are intrinsically dynamic in nature since they are related to electronic and atomic structures that  evolve with time. The characteristic time scales span from a few femtoseconds, in the case of electronic processes, to a few tens or hundreds of femtoseconds, as in the case of atomic and molecular processes.  Furthermore other phenomena, which control the behavior of critical systems, may happen at relatively longer time scales, ranging from a few picoseconds to a few hundreds of picosecond or more. These include phe-nomena produced by phase transitions, such as the processes related to magnetic order or to superconductivity. The capability of measuring these processes at the relevant time scales will open completely new perspectives and analyses.  In particular, the direct observation of electronic processes, of structural dynamics and of dynamical crit-ical phenomena (such as phase transitions) represents an unexplored landscape in the study of condensed matter physics. These possibilities were already evident to the inventors of the first coherent sources of femtosecond opti-cal pulses.  Ultra-sort pulses of coherent light have generated remarkable scientific progress that was recognized in the 1999 the Nobel Prize for Chemistry awarded to Ahmed Zewail for his pioneering work on the application of ultra-short laser infrared spectroscopy to the study of the dynamics of chemical bonds. Currently available, coherent light sources emit radiation only in a limited wavelength range.  Their use is limited to optical and spectroscopic techniques in the infrared, visible and near-ultraviolet range, excluding all the meas-urements needing photons of energy higher that a few eV.  There is therefore a strong scientific need for a tunable light source with an energy range from the vacuum ultraviolet (VUV) to the X-ray with a stable and well-charac-terized temporal structure in the femtosecond and picosecond time domain.  To this end, international research is moving in three main directions: 1) laser driven light sources which use non-linear processes to create very high harmonics, 2) interaction between an ultra-short laser pulse and an electron bunch in an storage ring (laser “bunch-slicing technique”), 3) free electron lasers (FEL). The first two techniques are able to produce radiation pulses in the femtosecond time domain and in the soft X-ray region with low brilliance (i.e. a low useful photon flux on the material under investigation). In contrast FELs can produce light pulses with peak brilliance as much as ten orders of magnitude higher than the pulses generated in present third generation synchrotron light sources and with pho-ton energies spanning from the VUV to the hard X-ray, i.e. from about 10 eV (120 nm) to 10 keV (0.12 nm). 


Ultima modifica il Giovedì, 17 Ottobre 2013 09:25