Adsorbate-substrate interaction in photofragmentation of trimethylsilylfluoride [Si(CH3)3F] adsorbed on Cu(111) following SiK-shell excitation

The photon-stimulated desorption (PSD) of fragment ions from adsorbed and condensed layers of trimethylsilylfluoride [Si(CH3)(3)F] with various layer thickness on Cu(111) has been studied using synchrotron radiation around the Si K-edge. The X-ray absorption near edge structures (XANES) spectra, desorption mass spectra and excitation-energy dependencies of desorption yields were measured for mono- and multi-layer surfaces. It was found that only lighter species, namely, H+, CH3+, and F+ were desorbed from the monolayer. The F+-yield spectra showed selective desorption at the transition from the Si 1s to an antibonding sigma(Si-F)*. In contrast, massive fragments, for example Si(CH3)(3)(+) became major desorbates from the multilayer. The suppression of the heavier fragments from the monolayer is explained by the efficient reneutralization by the strong coupling with the metal substrate. For the multilayer, most of the desorption spectra showed relative enhancement above the Si 1s threshold and less selectivity. We found that the surface fragmentation/desorption pathways are changed markedly depending on the layer thickness. (C) 1999 Elsevier Science B.V. All rights reserved.