The covalent attachment of acetophenone on Si(100) has been investigated using high-resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) calculations. The HREELS spectrum of the chemisorbed monolayer shows the absence of the C=O stretching mode around 1687 cm(-1) coupled with the retention of all vibrational signatures of a phenyl ring. The experimental results unambiguously demonstrate a [2 + 2]-like cycloaddition mechanism for acetophenone chemisorption on Si(1 00) through the binding between the C=O group and a Si dimer, consistent with the prediction of DFT calculations. (c) 2005 Elsevier B.V. All rights reserved.