Decay of "autoionizing" core-hole states in adsorbed CO molecules

The decay of resonantly excited ($1s\to 2\pi$) C and O $1s$ core-hole states, called autoionization in the gas phase, was studied for absorbed CO molecules by means of angle-resolved Auger spectroscopy for three different systems [CO/Cu(100), CO/Ni(111), CO+K/Ni(111)] which represent rather different bonding configurations. It is shown that the final states resulting from the $2\pi$-participant decay are, unlike the situation for the isolated molecule, not identical to the final states reached in valence photoemission. This is due to the coupling of the $2\pi$ level to the substrate valence band and to the dynamics of the screening process. For CO/Cu(100), whose decay spectra differ from those observed for other systems, a decay of two different initial states is proposed.