Electron stimulated desorption from selected phases of physisorbed CO/graphite: evidence for structure sensitive desorption dynamics

We have investigated electron stimulated desorption of , , and ions from selected surface phases of oriented CO molecules physisorbed on graphite using electron energies in the range 0 - 70 eV. In the case of the negative ions we observe, at high coverage, several dissociative attachment resonances below an electron energy of 18 eV, which are absent from the low-coverage (1 - 3 ML) regime. By contrast, a previously unobserved resonance at 19.5 eV in the yield is apparent only in the monolayer. We also find that the onset of positive ion desorption varies between the different monolayer phases selected and we attribute this behaviour to the different orientation of the physisorbed molecules in the selected phases. These results indicate that structural effects influence the desorption process in a subtle, but measurable way.