Tracking electrons and atoms in a photoexcited metalloporphyrin by X-ray transient absorption spectroscopy

Simultaneously tracking electronic and molecular structures of a photoexcited metalloporphyrin, present for only 200 ps in a dilute solution, has been realized using X-ray transient absorption spectroscopy (XTA). Using laser pulses as excitation sources and delayed X-ray pulses as probes, we were able to identify the excited state electronic configuration of a nickel porphyrin as singly occupied 3d_x2-y2 and 3d_z2 molecular orbitals (MOs) with an energy gap of 2.2 eV, and energy shifts 4p_z MOs to 1.5 eV higher relative to that of the ground state, and an expanded porphyrin ring characterized by lengthening of Ni-N and Ni-C bonds. Moreover, kinetic XTA signals at different X-ray photon energies demonstrate the capability for acquiring the correlation and coherence between different optically excited states with the same technique. These results provide guidance for theoretical calculations as well as insightful understanding of optically excited states that play important roles in photochemical processes.