FIELD-ION APPEARANCE ENERGY SPECTROSCOPY OF CO+ ORIGINATED FROM RH(111) AND AU(111) SURFACE STEP SITES

Field ionization of CO at step sites of Rh(111) and Au(111) was studie d with field ion microscopy and mass-to-charge resolved field ion ener gy spectroscopy. Using a newly developed retarding potential technique , absolute values for CO+ field ion appearance energies were derived f rom measurements of the high energy onset of ion retardation curves an d from in situ measurements of the work function of the retarding elec trode. From analyses of the CO+ field ion appearance energies, field i onization of CO on Rh and Au surfaces was found to proceed by field de sorption. Applying a thermionic cycle, field-dependent values of the b inding energy for CO on Rh(111) and Au(111) were derived. For comparis on, CO field ionization was also studied at a passivated Rh surface. T he mechanism of field ion imaging of metal surfaces with CO+ ions and the field-dependent chemisorption of CO on different metal surfaces is discussed on the basis of experimental results and theoretical calcul ations.