Simulation of ultrafast excited-state dynamics and elastic x-ray scattering by quantum wavepacket dynamics.

Simulation of the ultrafast excited-state dynamics and elastic X-ray scattering of the [Fe(bmip)] [bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-4-pyridine] complex is presented and analyzed. We employ quantum wavepacket dynamics simulations on a 5-dimensional potential energy surface (PES) calculated by time-dependent density functional theory with 26 coupled diabatic states. The simulations are initiated by explicit inclusion of a time-dependent electromagnetic field. In the case of resonant excitation into singlet metal-to-ligand charge transfer (MLCT) states, kinetic (exponential) population dynamics are observed with small nuclear motion. In agreement with transient optical absorption spectroscopy experiments, we observe a subpicosecond MLCT → MLCT intersystem crossing and a subsequent decay into triplet metal-centered (MC) states on a picosecond time scale. The simulated time-resolved difference scattering signal is dominated by the MC component, for which the structural distortions are significant. On the other hand, excitation into MC states leads to ballistic (nonexponential) population dynamics with strong nuclear motion. The reason for these ballistic dynamics is that in this case, the excitation occurs into a nonequilibrium region, i.e., far from the minimum of the MC PES. This results in wavepacket dynamics along the principal breathing mode, which is clearly visible in both the population dynamics and difference scattering. Finally, the importance of decomposing the difference scattering into components by electronic states is highlighted, information which is not accessible from elastic X-ray scattering experiments.