Dissociative electron attachment and electronic excitation in Fe(CO)₅

In a combined experimental and theoretical study we characterize dissociative electron attachment (DEA) to, and electronically excited states of, Fe(CO)₅. Both are relevant for electron-induced degradation of Fe(CO)₅. The strongest DEA channel is cleavage of one metal-ligand bond that leads to production of Fe(CO)₄^-. High-resolution spectra of Fe(CO)₄^- reveal fine structures at the onset of vibrational excitation channels. Effective range R-matrix theory successfully reproduces these structures as well as the dramatic rise of the cross section at very low energies and reveals that virtual state scattering dominates low-energy DEA in Fe(CO)₅ and that intramolecular vibrational redistribution (IVR) plays an essential role. The virtual state hypothesis receives further experimental support from the rapid rise of the elastic cross section at very low energies and intense threshold peaks in vibrational excitation cross sections. The IVR hypothesis is confirmed by our measurements of kinetic energy distributions of the fragment ions, which are narrow (∼0.06 eV) and peak at low energies (∼0.025 eV), indicating substantial vibrational excitation in the Fe(CO)₄^- fragment. Rapid IVR is also revealed by the yield of thermal electrons, observed in two-dimensional (2D) electron energy loss spectroscopy. We further measured mass-resolved DEA spectra at higher energies, up to 12 eV, and compared the bands observed there to resonances revealed by the spectra of vibrational excitation cross sections. Dipole-allowed and dipole/spin forbidden electronic transitions in Fe(CO)₅-relevant for neutral dissociation by electron impact-are probed using electron energy loss spectroscopy and time-dependent density functional theory calculations. Very good agreement between theory and experiment is obtained, permitting assignment of the observed bands.