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X-ray emission (X.e.) and X-ray photoelectron (X.P.) spectra have been recorded for sodium and potassium hexacyanoiron(I1) and also for potassium cyanide. Core-orbital photoelectron data enables the X-ray emission spectra and the valence-band photoelectron s ectrum to be aligned on a common energy scale. Peak coincidence in X.p. and X.e. spectra of the Fe complex identified the least tightly bound orbitals as predominately Fe 3d and showed that the group orbitals with the next highest ionisation energy had not only N 2 p and C 2 p (as in CN-) character but also Fe 3d and Fe 4p character due to covalent ligand-iron bond formation. Relative peak intensities in the X.P., and absences in the X.e., spectra indicated that the main peak in the valence-band region was composed of 2s orbitals (presumably both C and N) but that the ligand molecular orbitals giving rise to this peak played little part in bonding to iron. Comparisons of molecular-orbital ionisation energies and atomic-orbital coefficients with theoretical calculations showed only qualitative greement.

The interpretation and use of x-ray photon spectra of substances containing second row elements has utilized a number of theoretical models. These models may be divided into three basic categories, namely, the isolated atom model, various molecular models, and a number of solid state models, it is the purpose of this paper to examine critically the validity and limitations of molecular models for interpreting published x-ray photon spectra and spectra obtained by this group on chlorine and sulfur bearing substances.Chlorine and sulfur bearing substances were chosen for at least three important reasons. First, a great deal of published experimental data already exists on the Kα, Kβ, and L2, 3 transitions of these substances. Second, motivated in part by the long standing controversy concerning possible 3d orbital participation in the bonding of second row elements, there are extensive quantum mechanical calculations for ions containing sulfur and chlorine via simple molecular orbital concepts. Thirdj the availability of accurate photoelectron spectroscopic data on these substances now permits a detailed quantitative comparison of x-ray photon transitions with quantum mechanical calculations.Detailed evaluation along these lines indicates that for many substances the theoretically calculated energy values are frequently within a few electron volts (or less) of the experimentally observed energies. This study, therefore, tends to substantiate a viewpoint suggested by some recently; namely, that for many substances the starting point in interpreting most of the basic features of soft x-ray spectra should be based upon molecular bonding approaches.