The adsorption configuration of furan on Si(100)-2x1 at 125 K has been investigated using x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), high resolution electron energy loss spectroscopy (HREELS), and semiempirical molecular orbital (MO) calculation. A chemisorbed furan species is identified, which does not desorb until 300 K. Our results clearly demonstrate the covalent attachment of furan onto Si(100), possibly through a [4+2] cycloaddition reaction. Based on the frontier molecular orbital (FMO) theory and work function measurements, an electron donation mechanism from furan to Si(100)-2x1 is proposed to be involved in the cycloaddition reaction. (C) 2001 American Institute of Physics.