Adsorption of CO on the (110) plane of tungsten electron impact, thermal desorption, and work function measurements

Abstract

The adsorption of CO on the (110) plane of tungsten has been studied using electron impact desorption, thermal desorption, and work function measurements in a single apparatus combining these various techniques. It is concluded that a single molecular adsorption state exists at 20–250 K (virgin-CO). At 300–400 K, 60% of the low temperature layer desorbs, the remainder converting principally to a beta-1 state, which has very small electron impact cross section; in addition to beta-1 an O + yielding state, which we call beta-precursor is formed. The beta-1 state is stable to 900 K, where some desorption and conversion of the remaineder to a beta-2 state occurs. The O + yielding state decays with increasing T and is gone at 800 K. Readsorption on beta-1 leads to two types of adsorption states called alpha and gamma, which seem to be site specific. Electron impact desorption yields mostly CO + and CO for virgin, O + for beta-precursor, and CO + and CO for the readsorption states. There is no isotopic mixing in virgin or in readsorbed CO, nor does readsorbed CO exchange with beta-1 or beta precursor. There is complete isotopic mixing in beta desorption. In addition, massive EID creates another state, characterized by a large dipole moment, also yielding O + in EID. This state can be converted to beta-1 by heating to 400 K. The total disappearance cross sections for the various states are virgin- CO5 × 10 −17 cm 2; γ- CO 1.6 × 10 −16 cm 2; α- CO 5 × 10 −17 cm 2; β- precursor 6 × 10 −18 cm 2 and 1.2 × 10 −19 cm 2; EID induced state 8 × 10 −18 cm 2. In addition, cross sections for ion production are determined and found to be several orders of magnitude less than total disappearance cross sections. These results, and Leed and coverage data obtained in parallel investigations are used to formulate models of the various adsorption states. It is concluded that virgin and readsorbed CO are molecular and beta-precursor and beta dissociated, although strong interactions between C and O remain. The electron impact desorption of physisorbed CO was investigated and found to yield C +, O +, and neutral CO, but very little CO +. These results suggest primary dissociation of CO by electron impact, and desorption of neutral physisorbed CO by the energetic fragments. Physisorbed CO +, although undoubtedly created, lies on the attractive part of its potential curve relative to the surface, and thus does not desorb as CO +.