  Photo-induced switching from the low-spin state to the high-spin state is
studied in a model of spin-crossover materials, in which long-range
interactions are induced by elastic distortions due to different molecular
sizes the two spin states. At a threshold value of the light intensity we
observe nonequilibrium critical behavior corresponding to a mean-field spinodal
point. Finite-size scaling of the divergence of the relaxation time is revealed
by analysis of kinetic Monte Carlo simulations.
