ELECTRON-STIMULATED DESORPTION FROM SELECTED PHASES OF PHYSISORBED COGRAPHITE - EVIDENCE FOR STRUCTURE SENSITIVE DESORPTION DYNAMICS/

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