Competitive charge- and energy-transfer processes following core ionization in the Na- CO cluster

Anion-molecule clusters constitute a very suitable class of systems for studying intermolecular (interatomic) charge-transfer (CT) processes following core ionization. A weakly bound electron of the anion in these clusters can be easily transferred to the core-ionized molecule. The screening effect of this electron may have a dramatic impact on core-level spectra and even account for a breakdown of the quasiparticle picture of core ionization. This is demonstrated here by calculating the O1s(-1) and C1s(-1) core ionization spectra of the Na- CO cluster using an ab initio fourth-order Green's-function method. Interestingly, along with the CT processes in this cluster there exist also very efficient energy-transfer (ET) processes favored by the low excitation energies of Na-. These ET processes constitute an appreciable part of the electronic excitations following core ionization of Na- CO and exert thereby a strong influence on the spectra studied. The spectral features attributed to the ET processes are as pronounced as those attributed to the CT processes. Major differences in the behavior of CT and ET satellites as a function of the anion-molecule separation are found and explained. We compare also the O1s(-1) core ionization spectra of the Na- CO and Na- H2O clusters. Along with a certain similarity, these spectra exhibit substantial differences which are essentially attributed to the distinct cluster geometries.