Experimental and theoretical NEXAFS/XPS study of the room-temperature adsorption of acetonitrile onSi(001)−2×1

Combining synchrotron radiation N $1s$ x-ray photoemission and x-ray absorption spectroscopy to density functional theory calculations of electron transition energies and cross sections performed on silicon clusters, we have re-examined the issue of acetonitrile adsorption on single- and two-domain $\mathrm{Si}(001)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ surfaces, taking into consideration the various adsorption models proposed in the recent theoretical works. It is shown that at 300 K and saturation coverage, the molecule is chemisorbed both under nondissociated and dissociated forms. The nondissociative adsorption mode results from a $\mathrm{di}\text{\ensuremath{-}}\ensuremath{\sigma}$ bonding involving the cyano group. The use of a vicinal surface allows us to show that the side-on (a cycloadditionlike product on the Si dimer) is the majority species of $\mathrm{di}\text{\ensuremath{-}}\ensuremath{\sigma}$ type. The datively bonded molecule (end-on) is not observed at 300 K. The molecule also dissociates on the surface, under the form of a cyanomethyl plus a silicon monohydride, a model which has not been so far proposed by theoretical works.