Adsorption of CO on a Tungsten (100) Surface

The room-temperature adsorption of carbon monoxide on the (100) surface of tungsten has been studied using the techniques of low-energy electron diffraction, flash desorption, and work-function measurement. CO is found to exist in two binding states whch have been labeled βL and βH and is desorbed from these two states at ∼1150° and ∼1380°K, respectively. At room temperature, CO appears to adsorb into the βL state, and is associated with a work-function increase Δφ which is approximately linear with coverage and amounts to +0.43 eV at saturation—a coverage of about 1.0×1015 molecules/cm2. CO in the βL states can be converted into the βH state by heating to 900°—1000°K. Associated with the latter is an ordered c(2×2) structure which, at maximum coverage—about 5.0×1014 molecules/cm2—exhibits a Δφ of −0.16 eV with respect to the clean surface. Adsorption of CO to saturation on the c(2×2) structure appears to return the surface to a condition indistinguishable from that due to adsorption on the clean surface. A possible model for explaining this behavior is proposed.