James P. Kercher

From iron pentacarbonyl to the iron ion by imaging photoelectron photoion coincidence.

The dissociation dynamics of internal energy selected iron pentacarbonyl cations, Fe(CO)5(+), have been investigated using the imaging photoelectron photoion coincidence (iPEPICO) spectrometer at the Swiss Light Source. The molecular ion loses all five carbonyl ligands in sequential dissociations in the 8.5-20 eV photon energy range. The Fe(CO)5(+) parent ion is metastable at the onset of the first dissociation reaction on the time scale of the experiment. The slightly asymmetric and broad daughter ion time-of-flight distributions indicate parent ion lifetimes in the microsecond range, and are used to obtain an experimental dissociation rate curve. Further carbonyl losses were found to be fast at threshold. The fractional parent and daughter ion abundances as a function of the photon energy, that is, breakdown diagram, as well as the dissociation rates for the first CO loss were modeled using the statistical Rice-Ramsperger-Kassel-Marcus (RRKM) and statistical adiabatic channel model (SSACM) theories. The excess energy redistribution in the products was also taken into account in a statistical framework. The 0 K dissociative photoionization thresholds for the five carbonyl-loss channels were found to be 9.015 ± 0.024 eV, 10.199 ± 0.027 eV, 10.949 ± 0.033 eV, 12.282 ± 0.39 eV, and 13.821 ± 0.045 eV for the processes leading to Fe(CO)4(+), Fe(CO)3(+), Fe(CO)2(+), Fe(CO)(+), and Fe(+), respectively. The iron cation thermochemistry is well-known, and these onsets connect the bare metal ion to the other fragment ions as well as to the gas phase neutral Fe(CO)5.