Selective bond breaking in adsorbates by core excitations

We survey recent experiments which show that, by excitation of specific core hole states in adsorbates on metal surfaces, selectivity can be obtained in the breaking of bonds. For processes leading to the desorption of ions, strong selectivity arises from neutralization by charge transfer during desorption, which is strongly affected by the coupling to the substrate. However, except for complex multiply excited core excitations ions constitute minority channel; for lower core excitations mostly neutrals are found to desorb. After development of efficient detectors for neutral molecules and fragments we have been able to show that even here selectivity can be found for certain primary excitations for certain bonds to break, even for such simple test systems as N₂ and CO on transition metal surfaces. An analysis of the decay electron spectra seemed to indicate that this may be connected with preferential decay of certain primary excitations into different final state channels. However, since more data have been accumulated and results for N₂ on Ru(001) can now be compared with N₂ on Ni(111), and those of N₂ with CO on Ru(001), it has become clear that even here the effect of the surface by preferential quenching of certain excitations through coupling to the substrate is the overriding factor. Thus the surface influence on selective bond breaking is corroborated for core excitations as well.