Appearance energy spectroscopy of CO+ field ions emitted from W(111)

Abstract

Field ion appearance energy spectroscopy of CO+ field ions emitted from a few surface sites of the W(111) plane was performed. Using a newly developed retarding potential technique, absolute values for the CO+ appearance energies were derived from mass-to-charge resolved retarding potential analyses of field ions of the carbon monoxide supplied as an imaging gas in the field ion microscope. From analysis of the CO+ field ion appearance energies, field ionization of CO on the W(111) surface was found to proceed by field desorption. Applying a thermionic cycle, field-dependent values of the binding energy for CO on W(111) at high coverages were obtained. A comparison of the results for CO/W(111) with data for the field-free binding energy of CO on rough W planes, measured independently, is made. The 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 calculations.