On the initial and final state rules for predicting near-edge X-ray absorption intensities

Quantum chemical calculations for the first strong core-to-pi* transition in a series of carbonyl compounds are analysed for the purpose of testing the initial and final state rules in X-ray absorption spectroscopy. We study oxygen and carbon X-ray absorption spectra of a carbonyl group attached to hydrocarbon backbones and adsorbed on a copper surface modelled by clusters of metal atoms. We find, respectively, a weak and strong attenuation of absorption intensities in the two types of compound. For the hydrocarbons only a weak correlation with ground state populations could be found for compounds with identical binding to the carbonyl group. For the carbonyl-copper compounds the strong reduction of oscillator strengths related better to the delocalization of excited state pi* orbital than to the ground state p-population, thereby supporting a final state rule. (C) 1997 Elsevier Science B.V.