Theory of Ultrafast Low-To-High Spin Crossover in Divalent Iron Systems*

A theory is developed for the ultrafast low-to-high spin crossover in divalent iron. As soon as photoexcitation to the metal-to-ligand charge transfer (MLCT) ¹A₁ state from the ¹A₁ metal centered (MC) state occurs a sub 100fs decay to the ⁵T₂ iron MLCT through the ³T₁ MLCT state and, finally, a transition between MLCT and MC ⁵T₂ states occurs. A fundamental Hamiltonian is constructed but the most important key to a full and fast transition are the damping mechanisms. The significant energy gap between the MLCT states require a rate damping approach centered on phonon movement to fully activate the ⁵T₂ MLCT state but a novel damping approach, Boltzmann damping, is required to overcome the MLCT to MC gap. Whereas destructive interference suppresses spin-orbit-mediated transitions from MLCT to MC states, the Boltzmann damping connects high-energy MLCT states to low-energy MC states of the same symmetry.