Monte Carlo simulation for the photo-induced low-spin to high-spin state transitions - Difference and similarity with the thermal transition

The photo-induced cooperative low-spin (LS) to high-spin (HS) conversion phenomena are studied on the basis of a model in which the interaction between complexes is mediated by the lattice distortion (LD model). The relaxation rate of HS state decreases cooperatively as the fraction of it increases, since the potential barrier becomes high when the HS energy is lowered by the interaction. A Monte Calro method is developed to examine the LD model in the two- and three-dimensional cases. The existence of the: threshold light intensity and the incubation period before the steep conversion, as observed in experiments, are demonstrated. The incubation time corresponds to the nucleation time of the HS domain. It is pointed out that the nucleation barrier is surmounted by the fluctuation caused by the repetition of excitation and de-excitation, instead of the energy fluctuation in the usual temperature induced transition. But when an effective temperature is defined properly, the time scale of incubation period can be estimated following the usual domain formation theory, and it grossly accounts the MC results in three-dimensional cases.