A theoretical modelling approach using small and large silicon clusters respectively has been employed in the investigation of the chemisorption state of benzene on the Si(111)-7 * 7 surface. Three possible binding states were investigated at both HF and DFT levels as well as ONIOM (HF:AM1) and ONIOM (B3LYP:AM1) methods. 1,4 Di-σ state was found to be the energetically preferred state. The size effect of cluster has to be considered for the unstable mono-σ binding state. B3LYP method has much more accuracy than HF method in adsorption energy. Frequency calculation results, matching very well with the HREELS observations, further confirm the 1,4 binding state for benzene chemisorption. No transition state was found from mono-σ bonding state to 1,4-binding state by our theoretical calculation, so the adsorption mechanism of benzene on Si(111)-7 * 7 surface was proposed. It is that firstly benzene molecules are weakly trapped into a mono-σ bonding precursor state, which subsequently decay into a stable di-σ bonded chemisorbed state.

Key words: ADSORPTION, BENZENE, SILICON, CONFIGURATION, CLUSTER COMPOUND