Adsorption of Carbon Monoxide on Tungsten. Abundances, Dipole Moments, and Sticking Coefficients

Abstract

The relative abundances, relative dipole moments, and absolute sticking coefficients of virgin, alpha, and beta CO adsorbed on tungsten have been determined by a combination of work function vs coverage measurements with step-desorption spectra and sticking-coefficient measurements. The work-function data were obtained in a field-emission tube with an annular CO source, permitting uniform dosing of the tip. The other measurements were performed in a tube in which a CO source illuminated a tungsten ribbon. A movable field emitter serving as detector could be raised in front of this ribbon to receive CO from the latter in desorption or reflection. This arrangement represents a new method of determining sticking coefficients, free from the ambiguities of surface roughness and wall ad- or desorption, usable to 20°K or less. The dipole-moment ratios were Pα/Pv = −0.68; Pβ/Pv = 0.68 for the initial, and Pβ/Pv = 1.0 for the terminal portion of the beta layer. The relative abundances were total beta/total virgin = 0.55; virgin converted to beta/beta from virgin conversion = 2.1; alpha/total beta = 1.2. Initial sticking coefficients were close to unity up to 700°K, and decreased rapidly at coverages (relative to the maximum amount of adsorbate retained at that temperature) of 0.6–0.4.