Study on the dynamics of photo-induced structural change and valence change phenomena

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In 1999, we discovered a reversible ultraviolet-laser-light-induced phenomenon at room temperature in Eu_2O_3 for the first time. Since the discovery, we have found similar phenomena for many substances.Experimental studies have been done about the mechanism and dynamics of the ultraviolet- laser-light-induced structural change and valence number change phenomenon for several rare-earth metal oxides (Eu_2O_3,Sm_2O_3), several metal oxides (Y_2O_3,SiO_2,Al_2O_3, anatase TiO_2), quantum paraelectric (SrTiO_3) and several AgI-oxide nanoparticle composites, using bulk-, film- and nanocrystal specimens. Especially, we noted that the size of specimen system may determine the dynamics. All of the above specimens showed clearly both the reversible photo-induced spectral change and the memory effect at room temperature. Such reversible phenomenon may well yield materials for white-light-emitting devices and erasable optical storage. Theoretically, particle-size reduction may give rise to the confinement of excitation waves and long-wavelength phonon modes in the particle in the particle. However, the size dependence on the dynamics has been not observed in the experiments.The obtained results are explained by the exciton theory. The exciton instability induced by photo-irradiation leads to lattice decomposition into an electron centre and a hole centre, if the exciton is formed in the bulk. On surfaces, the hole centre is emitted from the surface, thus leaving only the electron centre. Therefore, such exciton instability may result in oxygen desorption at specimen surfaces. The equilibrium may be realized in a system composed of specimen and a radiation field.