The structure of the carburized W(001) surface

Abstract

The carburized surface of W(001) has been of interest because of the moderating effect of the carbon on the catalytic properties of the W surface. We have studied the carburized surface of W(001) formed by dissociative adsorption of ethylene using Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and alkali ion scattering. The previously published sequence of LEED patterns and associated coverages were reproduced. The intensity of the Li +/C single scattering intensity was monitored as a function of exposure for various ion incident angles and correlated to the C/W Auger intensity ratio. Analysis of this data showed that C atoms adsorb above the first W layer at all coverages, including that associated with a (5×1) LEED pattern generated by adsorption at 1500 K. The structure has been obtained by analysis of the incident angle dependence of the Li + on W single scattering intensity for the clean and carbon covered surfaces. No major reconstruction of the carburized surface occurs, even for the (5×1). The prominent C single scattering peak, the incident angle dependence and K + multiple scattering features indicate that the C resides at or near the four-fold hollows at all coverages. Analysis of the incident angle associated with shadowing of W by C demonstrates that the C atoms are 0.5 Å above the plane of the first layer W. The LEED (5×1) pattern is attributed to ordering in the C overlayer and a possible small amplitude periodic strain induced in the first layer W by the C.