Energy distributions of field emitted electrons from tungsten in the presence of adsorbed CO

Energy distributions were measured on the (110), (100), (111), and (210) planes under various conditions of CO adsorption. Resonances were seen for the low temperature layer at −7 to 7.5 eV below vacuum on all planes except (100). A comparison of the widths and maximum amplitudes of the resonances shows good agreement with the theory of field emission from adsorbate covered surfaces. On heating, the resonances disappeared at ≤350 K corresponding to conversion of the low temperature states to beta-precursor states. Readsorption on beta or beta-precursor layers led to resonances at −7 eV on (210) and (111), but not on (110) or (100). The resonances are interpreted as arising from interaction of CO–π* orbitals with tungsten d-t2g orbitals in adsorption via the C atom on single W atoms. Disappearance of the resonances on heating is interpreted as activated rearrangement to new bonding configurations-for instance, bridge bonding via C-sp2 orbitals. The intensity of the resonances increases initially with coverage but, on all planes except (210), goes through a maximum and vanishes for θ<1. This is interpreted as interference of neighboring CO molecules, resulting in the squeezing out of charge from the labile-π orbitals responsible for the resonance. The details of intensity vs θ curves on different planes support this model. On the (100) plane, a resonance at −6.9 eV appeared at 600 K and is attributed to W–O bonding. A comparison of the temperature behavior of this resonance for O and CO covered emitters leads to the conclusion that CO on the (100) plane is rearranged but not dissociated up to at leat 820 K.