Direct access to the photo-excitation and relaxation terms in photo-switchable solids: non-linear aspects

We investigate the photo-excitation process and the relaxation of the photo -excited state in photo-switchable solids, such as spin transitions systems . We follow the competition between the relaxation and the photo-excitation (up and eventually down) processes. The kinetic experimental curves are tr eated so as to derive the various contributions to the master equation, in terms of a single macroscopic variable: the population of the photo-excited state. The experimental examples are taken from [FexM1-x(btr)(2)(NCS)(2)]. H(2)0, with M = Co, Ni, Zn. They give evidence for a non-linear character o f the relaxation at all temperatures, including the tunnelling regime, and for non-linear behaviours of the photo-excitation rates. (C) 2001 Elsevier Science Ltd. All rights reserved.