Dynamical model for spin-crossover solids. II. Static and dynamic effects of light in the mean-field approach

We have introduced a photoexcitation contribution in the dynamical model we previously presented (see previous paper), as an additional term in the rate transition, in order to reproduce the light induced thermal hysteresis effect, experimentally observed in the spin-crossover materials. The set of motion equations governing the evolution of the high-spin (HS) fraction of cooperative spin-crossover systems under light, is obtained. The mean-field treatment leads to a macroscopic master equation previously given on a phenomenological basis. With both the $\mathrm{HS}\to$ LS (low-spin) and $\mathrm{LS}\to$ HS transition rates terms, a model of general use is derived. This model describes the static and dynamic properties under light. We have investigated this model analytically, as far as possible. The properties under light involve two instabilities different in nature: the high-temperature instability, chiefly entropy driven, and the low-temperature one, light induced. We have determined the conditions for these instabilities to occur, and have examined their possible collapse. The described phenomena are discussed with respect to the available data on the photoinduced bistable regime: thermal hysteresis loops, optical hysteresis loops, and transient regimes. The kinetic dependence of the photoinduced hysteresis loops is calculated.