MANY-BODY GREENS-FUNCTION STUDY OF THE VALENCE-BAND FORMATION OF POLYOXYMETHYLENE

With the aim of searching for conformational signatures in valence X-ray photoionization spectra (XPS) of polyoxymethylene, Many-body Green's Function calculations have been conducted at the second-order aid diagonal 2ph-TDA levels of approximation on simple oligomers CH2-O-(CH2-O-)(x)-CH3 where x = 1, 2, 3, and 4. Variations observed in the shape of spectral bands are related to ab initio band structure calculations carried out on ideal infinite stereoregular polymers. These calculations, as well as photoionization intensities and line positions, give evidence of the fundamental role in conformational signatures of long-range interactions such as methylenic hyperconjugation, or pi(CH2) and sigma(CH2) ''through bond'' interactions. Many-body effects are analysed in terms of the combined influence of the pole strength factors together with the orbital relaxation, pair removal and pair relaxation contributions. These effects are shown to result in a substantial breakdown of the molecular orbital picture in the inner valence O-2 region. Important relaxation effects are also observed in the O-2p outer valence region, and tend to enhance conformational signatures in this part of the spectrum.