Theory of ultrafast photoinduced low-to-high spin crossover in divalent iron systems

A theory is developed for the ultrafast low-to-high spin transition in divalent iron. Following the photoinduced metal-to-ligand charge transfer (MLCT), the $^{1}A_{1}$ MLCT state decays on a sub-100-fs timescale into the $^{5}T_{2}$ MLCT using a $^{3}T_{1}$ MLCT state as intermediary. Damping of nuclear motion is crucial in obtaining a full singlet-to-quintet transition mediated by the spin-orbit interaction. Destructive interference suppresses transitions to metal-centered (MC) states. The relaxation to the MC $^{5}T_{2}$ is a result of Coulomb scattering with the surroundings.