The main topic of this thesis is the study of the ultrafast photo-switching of photo-magnetic molecular materials showing transition between spin states. These molecular crystals are prototypes of molecular bistability between two distinct electronic states, HS and LS. The molecules can be switched between these two states by a light pulse. The emergence of ultrafast techniques allows us to study in real time these photo-switching processes and also the associated out-of-equilibrium dynamics down to the femtosecond scale (10-15s). We have combined probes sensitive to the change of electronic state, on the hand, and to structural rearrangements, on the other hand, in order to observe these photo-switching processes. The measurements of ultrafast transient absorption spectroscopy have been made using the laser plateform at the IPR. Complementary time resolved X-ray diffraction and absorption experiments have been performed on large facilities. The first part of this manuscript is focused on the photo-switching dynamics at molecular scale. It reveals a complicated interaction between electronic and structural degrees of freedom. The generation and damping of coherent optical phonons is identified as a key parameter in the trapping in HS potential. Several experiments on different compounds show the linear and local character of such ultrafast photo-switching. The second part of this thesis presents studies on the complete out of equilibrium dynamics. It reveals a cascading process with activation of elastic and thermal effects at different time scales. Cooperative processes following a light excitation are observed. These complexes dynamics are driven by propagating and diffusive process sensitive to the size of the sample. The study of nanocrystals yields high conversion and faster response to elastics effect than single-crystals. These studies further elucidate the out of equilibrium processes underlying the photo-induced phase transitions on time and length scales, from the molecule to material scale.
| Identifer | oai:union.ndltd.org:CCSD/oai:tel.archives-ouvertes.fr:tel-01016162 |
| Date | 27 June 2013 |
| Creators | Bertoni, Roman |
| Publisher | Université Rennes 1 |
| Source Sets | CCSD theses-EN-ligne, France |
| Language | English |
| Detected Language | English |
| Type | PhD thesis |
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