Electron impact ionization studies of ferrocene, cobaltocene, nickelocene, and magnesocene

Abstract

Negative and positive ions of ferrocene, cobaltocene, nickelocene, and magnesocene produced by collisions with monoenergetic electrons have been studied. Appearance potentials relative to the ionization potential of krypton have been determined for the products (C10H10M)+, (C5H5M)+, and M+. The ionization potentials derived from the present electron impact studies are in good agreement with recent photoelectron values. In all cases cyclopentadienyl anions were produced at low electron energies; however, their cross sections for formation differed by orders of magnitude. The most striking feature of the negative ion studies was the observation of a long-lived parent negative ion of nickelocene at thermal electron energies, and a second broader compound negative ion resonance at approximately 1 eV. The autodetachment lifetime for the second resonance decreases from 60 μsec at 0.5 eV to 20 μsec at 1.5 eV. Cobaltocene also exhibits two long-lived negative ion states in the energy region below one electron volt which are barely resolvable. Ferrocene forms a short-lived (τ ≳ 2× 10−14 sec) compound negative ion state at ∼0.5 eV. The negative ion results can be qualitatively explained by assuming that the captured electron occupies one of the empty e1g or e2u orbitals of the parent molecule.