Organized by the Institute of Experimental Physics, University of Bialystok and the Institute of Atomic Energy, Swierk under the auspices of the European Crystallographic Association; Committee of Crystallography, the Polish Academy of Sciences; and the Polish Neutron Scattering Society The School is financially supported in part by the International Union of Crystallography; the European Crystallographic Association; the European Office of Aerospace Research & Development; the Ministry of Science and Information Society Technologies; the Warsaw University of Technology; and the University of Bialystok.
		The mean-field macroscopic master equation approach to the kinetic properties of cooperative photo-switchable molecular solids. François Varret and Kamel Boukheddaden Laboratoire de Magnétisme et d'Optique CNRS-Université de Versailles, France Topics The lecture aims to introduce the basic concepts suited to describe the thermodynamic behavior of "photo-switchable molecular solids" (PSMS), i.e. solids which undergo photo-excitation and relaxation processes between interacting local states. It will be organized as follows : (I) Introduction to switchable molecular solids. Typical examples of PSMSs, viz. spin-crossover solids, photo-magnetic Prussian blue analogues and other photo-magnetic systems will be presented. The various switching processes (temperature, irradiation, pressure, magnetic field) will be illustrated and possible applications briefly outlined. (II) Static aspects The interacting two-level model (Ising-like) model will be presented, and its equilibrium properties briefly established. Typical experimental data will be discussed. (III) Kinetic aspects The Mean Field Macroscopic Master Equation (MFMME) will be defined, for the case of interacting two-level systems. The assumption of linear photo-excitation rates and the self-accelerated form of the relaxation rates will be briefly discussed. The MFMME is useful for investigating: - the static properties - in agreement with the canonical treatment - with their stability character, wich for instance governs the occurrence of hystereses at the thermal transition (entropy-driven). Both static and dynamic properties are described by the same equation. - the photo-stationary states of the system under steady irradiation, with their stability character, - the response of the system for all time dependences of the external parameters (temperature, irradiation, pressure, intense magnetic field). In particular, we shall focus on the non-linear character of the MFMME, due to the presence of interactions. The non-linear character is responsible - in addition to the hysteretic properties of the entropy-driven transition - for the light-induced instability and the subsequent hystereses and intensity threshold effects which occur when photo-excitation and relaxation rates compete against each other. A striking example, which can be modeled by the MFMME is that of the photo-induced transition in Prussian Blue analogue, which can be driven in both ways using the same wavelength, according to the chosen temperature in the thermal hysteresis loop. F. Varret et al, 5th Int. Conf on Solid State Chemistry, Bratislava, Slovaquie, 7-12 Juillet 2002 : "Examples of Molecular Switching in inorganic solids, due to temperature, light, pressure and magnetic field ", Proceedings in Pure Appl. Chem, 74 (2002) 2159-2168 F. Varret, et al, Int. Conf. on Molecular Magnetism (ICMM04 Tsukuba, Japan, 5-8 Oct. 2004, Invited conference) "Light-Induced instability and Intensity thresholds in Molecular switchable solids: the Mean-Field Macroscopic Master equation approach", Proceedings to appear in Polyhedron