MANY-BODY STUDY OF IONIC STATES IN ADSORPTION SYSTEMS - CL-SI(100)-(2X1)

In this Letter we study the excited states formed upon removal of one electron from the Cl-Si(100)-(2 x 1) adsorption system. Their excitati on energies, charge distributions and excitation mechanisms are discus sed by means of a many-body approach specially suited to take into acc ount the possibility of hole localization. The main emphasis is given to the threshold energies of Cl+ desorption. It is found that only som e one-hole excitations are sufficiently long-lived to survive all the way out to vacuum and thus lead to Cl+ desorption. This long life is c onferred by the presence of localized electron-hole pairs (eh) in the final state. The screening electron, placed in the antibonding level o f the bond, stabilizes both the primary hole and its partner hole in t he eh pair. Our calculated density of states for the adsorbate in the valence region agrees with the photoemission spectrum, in contrast wit h previous calculations.