Resonant Auger spectroscopy study of charge transfer phenomena in N 1s core-excited acetonitrile adsorbates on Si(001)-2 x 1

Acetonitrile (CH3CN) adsorbs on Si(001)-2 x 1 at room temperature under two forms, a cycloaddition-like adduct (Si-C=N-Si) and a pendent cyano (Si-CH2-C-N) resulting from the decomposition of the molecule. Resonant Auger spectroscopy has been used to study the excited-state-dependent electron transfer from the N Is core-excited molecular adsorbate to the silicon substrate, using the core-hole lifetime (similar to 6 fs) as an internal clock. It is shown that the pi(*)(C=N) NEXAFS state lies within the silicon bandgap because of a core-excitonic effect. Therefore no charge transfer of the excited electron to the substrate is observed. On the other hand the pi(*)(C=N) NEXAFS state is placed within the silicon conduction band. Excitation to this orbital leads to valence/Auger spectra in which both resonant and normal Auger contributions are observed. Therefore there is evidence for a charge transfer from the pendent C N to the silicon surface, on a timescale estimated to tens of femtoseconds. (c) 2006 Elsevier B.V. All rights reserved.