Band shape and vibrational structure in Auger spectra: Theory and application to carbon monoxide

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Abstract
A time-dependent approach to Auger spectra is presented and used to derive simple working equations for computing the vibrational broadening and the vibrationally induced shift of the peaks in the spectrum. The formulas give the explicit dependence of the vibrational envelope on the local details of the electronic potential energy surfaces of the intermediate and final states, providing interesting general insights which we discuss in detail. It is shown that, in polyatomic molecules, relevant interaction terms among different nuclear modes arise. The theory applies as well to other processes which involve a core ionized or core excited intermediate state like, for instance, x-ray emission or resonant Auger decay. As a test application, the double ionization spectrum of CO is computed by the Green’s function method, and the new equations, together with a two-hole population analysis of the pole strengths, are used to obtain theoretical Auger spectra. The experimental spectral profiles, characteristically shaped by the varying vibrational broadening and substantial energy shifts, are accurately reproduced, giving most peak positions to within a few tenths of eV. The results present very different vibrational effects for the carbon and oxygen spectra, showing the general inadequacy of interpretations based on vertical transition energies only. Hole localization in the dicationic states is discussed in the light of the population analysis results and used to estimate the electronic Auger transition rates.