Understanding resonant inelastic X-ray scattering experiments of diazines via quantum dynamics simulation.

We present a combined theoretical and experimental study of the three diazine isomers pyrazine, pyrimidine, and pyridazine by means of resonant inelastic X-ray scattering (RIXS) at the nitrogen K-edge, employing fully time-dependent quantum dynamics simulations to understand the role of nuclear motion in core-excited states. The RIXS process is simulated by wave-packet propagation in both the valence- and core-excited state manifolds, carried out with the (multilayer) multiconfigurational time-dependent Hartree [(ML-)MCTDH] method. We use linear vibronic coupling Hamiltonians with up to 22 electronic states and compare a full-dimensional (24-mode) and a reduced six-mode model. We find good agreement between experiment and theory for all three molecules. In particular, our study highlights the essential role of nuclear motions during the population of short-lived intermediate core-excited states. Specifically, we show that ultrafast non-adiabatic transitions induce symmetry distortion that leads to additional emission bands, while interstate vibrational dynamics lead to vibrational progressions in the inelastic scattering spectra. These results establish a dynamical picture of the RIXS process in diazines that emphasises the importance of including nuclear dynamics in the calculations of resonant Raman processes.