Photoionization study of Fe(CO)5 and Ni(CO)4

Photoionization yield curves from onset to 600Å, and ionization threshold values have been obtained for the ions $\text{Fe}{(\text{CO})}_5^{+}$, $\text{Fe}{(\text{CO})}_4^{+}$, $\text{Fe}{(\text{CO})}_3^{+}$, $\text{Fe}{(\text{CO})}_2^{+}$, $\text{Fe}{(\text{CO})}^{+}$, Fe⁺ and CO⁺ from iron penta-carbonyl, and for $\text{Ni}{(\text{CO})}_4^{+}$, $\text{Ni}{(\text{CO})}_3^{+}$, $\text{Ni}{(\text{CO})}_2^{+}$, $\text{Ni}{(\text{CO})}^{+}$, Ni⁺, and CO⁺ from nickel tetracarbonyl. From these curves, information on the ionization-fragmentation processes of Fe(CO)₅ and Ni(CO)₄ under photon impact, have been obtained. Differences in the threshold energies as determined by photon and by electron impact methods increase with the fragmentation. The appearance potentials and the heats of formation of the metal-ions, agree within 0.1 eV, with those calculated from thermochemical data. The average bond dissociation energy of the neutral molecule agrees within 0.02 eV, with the thermochemical values. The kinetic shift for the formation of Fe⁺ and Ni⁺ ions is very small. The Ni ion is most likely formed directly from the molecule ion rather than by a cascade fragmentation process.