Tracking the\nElectron Density Changes in Excited States:\nA Computational Study of Pyrazine

The development of X-ray free-electron lasers has enabled\nultrafast\nX-ray diffraction (XRD) experiments, which are capable of resolving\nelectronic and vibrational transitions and structural changes in molecules\nor capturing molecular movies. While time-resolved XRD has attracted\nmore attention, the extraction of information from signals is challenging\nand requires theoretical support. In this work, we combined X-ray\nscattering theory and a trajectory surface hopping approach to resolve\ndynamical changes in the electronic structure of photoexcited molecules\nby studying the time evolution of electron density changes between\nelectronic excited states and ground state. Using the pyrazine molecule\nas an example, we show that key features of reaction pathways can\nbe identified, enabling the capture of structural changes associated\nwith electronic transitions for a photoexcited molecule.