TEMPERATURE-PROGRAMMED DESORPTION AS A PROBE FOR ADSORBATE STRUCTURE - DESORPTION OF A MIXED TICL4, SICL4, CCL4 ADLAYER

The use of temperature programmed desorption mass spectrometry as an indirect probe for adsorbate structure is described in relation to the desorption of a coadsorbed TiCl4/SiCl4/CCl4 layer from W(100). During temperature programmed desorption of such a layer, the SiCl4 and CCl4 molecular desorption features are very nearly coincident, with desorption rate maxima at 178 K, while TiCl4 desorbs at 215 K, a significantly higher temperature. We suggest that SiCl4 and CCl4 adsorb on W(100) in a similar fashion through coordination to the surface via lone electron pairs on one or more chlorine atoms. TiCl4 forms a more stable adduct with W(100), which we propose involves donation of electron density from the surface to the electron deficient titanium center. SiCl4 and CCl4 have closed valence shells, and are unable to adopt such a structure. An important aspect of this work is that coadsorbed layers were investigated. This allows direct comparison of desorption temperatures since TiCl4, SiCl4, and CCl4 are necessarily desorbing from the same surface.