Abstract: Chemisorption states of CO on W(110)

Abstract

When CO chemisorbed at low temperature on W(110) is heated, a variety of binding states are exhibited. This investigation follows the changes in a chemisorbed layer from 20 K to the desorption temperature on a polished single crystal face 1.5×0.25 cm in ultrahigh vacuum better than 10−12 Torr (1.33×10−10 Pa). A closely positioned field-emission tip measures the thermal desorption for each temperature increment by a method due to Bell and Gomer. Alternatively, flash desorption spectra for specific molecular masses can be obtained with a mass spectrometer. Binding states of CO on the surface are characterized by the ions observed in electron impact desorption (EID) as well as the change produced in the surface work function as measured with a vibrating condenser arrangement. Decay of ion signals for large electron currents yields the total EID cross section useful for identification too. A uniform layer of virgin CO forms on adsorption at 20 K from an effusion beam. The work function increases by 0.6 eV. EID yields predominantly CO+. Heating to 300–400 K desorbs 60% of this layer with disappearance of CO+ and simultaneous appearance of O+ in EID. The work function meanwhile reverts to approximately the clean metal value. With further heating, O+ diminishes without thermal desorption, suggesting the formation of an intermediate β-precursor state from virgin CO with subsequent conversion to β-CO, β-CO then desorbs between 900 and 1200 K with complete isotope mixing. More evidence for binding-state mutation between 400 and 900 K was sought by readsorbing CO at 20 K on β- or β-precursor-CO heated to a specified temperature. The readsorbed CO, isotope labeled, was found to be accomodated in two states thermally desorbable in a continuous spectrum up to 400 K. Both yield CO+ on EID but are distinguishable from each other and virgin CO by the thermal desorption temperature, work function change, and EID cross section. The weaker binding state, previously unknown, was named γ CO to distinguish it from the more strongly bound α-CO. While the nature of the readsorption states did not depend on the temperature to which the β- or β-precursor- CO had been heated before readsorption, the amount of readsorption possible increased with that temperature. This fact suggests an increasing extent of some kind of conversion, possibly from β-presursor to β. In addition, the presence of α- and γ-CO was found to suppress O+ ions from the β precursor in EID. A small amount of conversion of α-CO into β precursor was detected with isotope labelling. Isotope effects in the EID total cross sections for virgin and β-precursor states are small. The total cross section for virgin 12C16O is 10% larger than for 12C18O. The ionic cross sections show more pronounced differences as shown by the ratio of ion currents from a mixture of equal amounts of each isotope labelled species. C16O+/C18O+ ?1.6 for virgin CO. 16O+/18O+?1.3 for β precursor. The CO+-yielding virgin state can be converted to an O+-yielding state by the impact of low energy electrons (∠200 eV). Work function, however, suffers little or no change in this conversion, quite unlike the thermal conversion to β precursor. Readsorption on a layer thus produced by electron impact yields essentially virgin CO rather than α- and γ-CO as shown by subsequent thermal desorption. The electron-induced O+-yielding state is thus different from the O+-yielding β or β precursor. If this O+-yielding state is subjected to prolonged electron impact, the O+ ion signal is reduced to a very small value, with a concurrent decrease in the work function. On heating, the O+ ion signal increases to a maximum at about 200 K and then decreases with rising temperature to zero at the β-CO desorption temperature.