Relative partial photoionization cross-sections and photoelectron branching ratios of the valence-bands of the group VIII metallocenes of iron, ruthenium and osmium

Abstract

Relative partial photoionization cross-sections have been measured for the valence-bands of bis-η5-cyclopentadienyl iron, ruthenium and osmium over the photon energy range 16–115 eV. In each case the a′1 -1 and the e′2 -1 bands show cross-section features attributable to nd resonant photoemission in the vicinity of np → nd giant resonant absorption. For Ru(η-C5H5)2 a large intensity increase is observed, while Fe(η-C5H5)2 and Os(η-C5H5)2 show cross-section minima due to interference effects, followed by much weaker maxima. In the case of Ru(η-C5H5)2 all the ligand ionization bands exhibit resonance features very similar to the metal based orbitals, with those observed for e″1 -1 ionization being the most intense. Even the e″1 -1 orbital, which by symmetry has no metal d character, exhibits such a resonance in its cross-section. For ferrocene and osmocene the e″1 -1 bands also exhibit the greatest degree of cross-section correlation with the d bands. Significant differences are found between the a′1 -1 and the e′2 -1, and between the e′1 -1 and e″1 -1 bands. These are attributed to the varying amounts of metal character and the different symmetries of the corresponding molecular orbitals. The implications of the relative intensity changes with regard to the use of He I/He II intensity ratios as an aid to spectral assignment are examined. Other intensity maxima in the d orbital cross-sections are tentatively assigned to shape resonances in one or more of the f-wave ionization channels. The cross-sections of the ruthenocene e″1 -1 bands cannot be adequately described by linear combinations of the e′1 -1 and metal band cross-sections. Relative partial photoionization cross-sections for the 4f ionization bands of Os(η-C5H5)2 are reported over the photon energy range 75–115 eV.