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

Abstract

In a combined experimental and theoretical study we characterize dissociative electron attachment (DEA) to, and electronically excited states of, Fe(CO). Both are relevant for electron-induced degradation of Fe(CO). The strongest DEA channel is cleavage of one metal-ligand bond that leads to production of Fe(CO). High-resolution spectra of Fe(CO) 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) 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) 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)-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.

Suppression of low-energy dissociative electron attachment in Fe(CO)5 upon clustering

“…2 shows the yields of monomeric anions (those containing one iron atom) from pure Fe(CO) 5 clusters. The data were taken on the CLUSTER setup and are compared with the anion yields from gas-phase iron pentacarbonyl [ 18 ], measured in effusive molecular beam with a similar trochoidal electron monochromator. The most striking difference is in the strongest fragment Fe(CO) 4 − : the sharp peak close to 0 eV disappears and the maximum shifts to 0.65 eV.…”

“… Red lines: Yield of anions containing one iron atom, following DEA to pure Fe(CO) 5 clusters as measured using the CLUSTER setup. Black lines: Anion yields from the gas-phase Fe(CO) 5 [ 18 ]. …”

“…We have evidence [ 18 ] that the cross section for the process ( Eq. 1 ) is strongly enhanced by the presence of a virtual state in the electron–Fe(CO) 5 scattering.…”

“…Interestingly, the disappearing band lies in energy range, where the excitation of the singlet Fe(CO) 5 * states prevails in step ( Eq. 4 ), while the band that pertains lies in the energy range for triplet excitations [ 14 , 18 ]. We do not have a good explanation of the origin of such a state-selective ligand stabilization by the argon support.…”

Electron attachment to hexafluoropropylene oxide (HFPO)

“…This process can lead to very different reaction channels than DEA or the positive ionization. 34,35 Several findings outlined above suggest that the DEA mechanism is a vertical electron attachment followed by a prompt dissociation, rather than an internal vibrational redistribution of the excess energy and statistical dissociation. These findings are the absence of the collisional stabilization in the swarm experiment and the fact that the relative abundances of the fragments are not at all correlated with the asymptotic energetics.…”