MS38-P02 Sub-na and sub-ps time resolved diffraction at CRISTAL beamline - SOLEIL synchrotronThe study of out-of-equilibrium dynamics in crystalline solids has become an important topic of modern condensed matter physics. Out-of-equilibrium states are obtained by irradiation with ultra-short laser pulses in the optical range, which induces electronic transitions on a timescale at which the lattice is considered to be frozen. Such photo-induced dynamics are studied experimentally in the so-called ‘pump-probe’ scheme, in which the sample is excited (pumped) by a fs optical laser pulse and probed by a pulse of an eventually different wavelength such as X-rays. Pulsed hard X-ray sources open the possibility to use diffraction and X-ray absorption to study photo-induced transitions in condensed matter under non-equilibrium conditions, and thus uniquely give access to precise information on the dynamics of atomic structures at the Å-scale.
In laser pump - X-ray probe experiments, the time resolution is in practice limited by the X-ray pulse duration. SOLEIL synchrotron, in its standard mode of operation, provides pulses of typically 80 ps FWHM . X-ray pulses as short as 10 ps and 100 fs are also produced, in the “low-α” mode of operation  and with the femto-slicing source , respectively.
The undulator-based diffraction beamline CRISTAL enables studies of ultrafast structural dynamics using those short X-ray pulses. The sample is excited by 800 nm, 25 fs FWHM laser pulses provided by a regenerative Ti:Sa amplifier (max. output power 6 mJ @ 1 kHz). The subsequent changes in the sample’s atomic structure are studied in the time domain Dt, by measuring its diffraction out of a monochromatic incident beam. The X-ray flux available at sample, which depends on the aimed time resolution, ranges from 6.108 ph/s for 7 keV, 70 ps FWHM X-ray pulses at a repetition rate of 1 kHz, to 1.106 ph/s for 7 keV, 100 fs FWHM X-ray pulses at the repetition rate 1 kHz. A 2D, gateable detector is used to collect the scattered intensities, which gives access to a large portion of reciprocal space for each single measurement. Various sample environnements have been made available, including a near-ambient temperature Peltier cell, a 5 K cryostat, and a 30 K N2/He blower.
The poster will present the instrumentation developped at CRISTAL beamline for pump-probe diffraction experiments, as well as recent exemples of measured ultrafast structural dynamics.
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Keywords: Diffraction, synchrotron, time-resolved