Fig. 4: Photo-excitation cycle of a single crystallite.

1: laser induced charge transfer promotes Low Spin (LS) molecules (not shown for clarity) residing in LS lattice (blue) to a hot High spin (HS) state (shape and color coded consistently with Fig. 3) in less than 100 fs. 2: vibrational cooling of hot molecules with LS lattice (color-coded consistently with Fig. 3) takes place on the picosecond time scale. 3: volume expansion of heated lattice in 100’s ps, and coexistence of LS molecules (not shown) in thermally dilated LS lattice (blue) with photo-switched HS molecules (red) in thermally dilated HS lattice (red). HS and LS lattices superimposed to emphasize their likely coexistence (only average volume measured with X-ray diffraction). 4: shift of the switching regimes: LS molecules residing in the ground state switch to the HS state according to LS/HS occupancy probabilities modified by lattice expansion; the delay between expansion (300 ps, or less for nano-crystallites) and the switching (ns time-scale), due to incompressible Arrhenius-type kinetics sketched in the insert. 5: equilibrium recovery within μs time scale.