Imaging the photophysics of ironpentacarbonyl with ultrafast x-ray scattering

We directly observe the early excited state gas-phase dynamics in the photodissociation of iron pentacarbonyl using ultrafast x-ray scattering. We recorded isotropic and anisotropic difference scattering signals following an ultrashort UV (266 nm) pump and hard x-ray (9.5 keV) probe. Applying the natural scattering kernels method we inverted the scattering signals to real space and recovered the pair density dynamics. We observe coherent periodic motion across several pair distances that is a precursor to the first CO loss, where each period contributes to the dissociation. Using the anisotropic scattering information we obtain evidence that the first CO loss has a preferential axial component.

This observation agrees with ab-initio excited state molecular dynamics simulations that were reported recently, which suggest that the dynamics are a consequence of periodically reoccurring non-adiabatic transitions from metal-ligand charge transfer to metal-centered states.

We further observed a second CO photodissociation by analyzing the pair density dynamics at the 2-4 angstrom region that correlates with the loss of Fe-C and Fe-O pair distances. We applied a kinetic model and obtained the thermal rate of the second CO loss, with an accuracy that is 6-fold higher than previous spectroscopy studies. The real-space time-resolved information we obtained allows for direct observation of motions in the coherent to thermal time window, which play a crucial role in determining the chemical properties of short-lived intermediates that are active in catalytic reactions.