The adsorption and binding of chlorobenzene (C6H5Cl) on clean and D-modified Si(111)-7 x 7 surfaces have been investigated using high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). On a clean surface, both chemisorbed and physisorbed C6H5Cl are observed at an adsorption temperature of 110 K. The HREEL spectra show direct evidence for the presence of both sp(2) and sp(3) carbon atoms in chemisorbed C6H5Cl molecules on Si(111)-7 x 7. Upon D-modification, the chemisorption of C6H5Cl decreases rapidly with increasing D-coverage (theta(D)). At theta(D) = 1/3 ML, only physisorbed chlorobenzene is detected, which strongly suggests the involvement of rest-atoms in the chemisorption of C6H5Cl. Combined with the scanning tunneling microscopy (STM) results by Chen [Surf. Sci. 340, 224 (1995)] showing the participation of adatoms in the binding, we propose that the 2,5-carbon atoms in C6H5Cl are stereo-selectively di-sigma bonded to a pair of adjacent adatom and rest-atom on the Si(111)-7x7 surface, yielding a 2,5-chlorocyclohexadienelike surface adduct. (C) 2000 American Institute of Physics. [S0021-9606(00)70610-6].