  Disentangling the dynamics of electrons and nuclei during nonadiabatic
molecular transformations remains a considerable experimental challenge. Here
we have investigated photoinduced electron transfer dynamics following a
metal-to-ligand charge-transfer (MLCT) excitation of the [Fe(bmip)2]2+
photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1- ylidine)-pyridine,
with simultaneous femtosecond-resolution Fe K{\alpha} and K\b{eta} X-ray
Emission Spectroscopy (XES) and Wide Angle X-ray Scattering (WAXS). This
measurement clearly shows temporal oscillations in the XES and WAXS difference
signals with the same 278 fs period oscillation. The oscillatory signal
originates from an Fe-ligand stretching mode vibrational wavepacket on a
triplet metal-centered (3MC) excited state surface. The vibrational wavepacket
is created by 40% of the excited population that undergoes electron transfer
from the non-equilibrium MLCT excited state to the 3MC excited state with a 110
fs time constant, while the other 60% relaxes to a 3MLCT excited state in
parallel. The sensitivity of the K{\alpha} XES spectrum to molecular structure
results from core-level vibronic coupling, due to a 0.7% average Fe-ligand bond
length difference in the lowest energy geometry of the 1s and 2p core-ionized
states. These results highlight the importance of vibronic effects in
time-resolved XES experiments and demonstrate the role of metal-centered
excited states in the electronic excited state relaxation dynamics of an Fe
carbene photosensitizer.
