Potential energy barriers for dissociative chemisorption of O₂ molecules on Si (001) clean surfaces were investigated using supersonic O₂ molecular beams and photoemission spectroscopy. Relative initial sticking probabilities of O₂ molecules and the saturated oxygen amount on the Si (001) surface were measured as a function of incident energy of O₂ molecules. Although the probability was independent on the incident energy in the region larger than 1 eV, the saturated oxygen amount was dependent on the incident energy without energy thresholds. An Si-2p photoemission spectrum of the Si (001) surface oxidized by thermal O₂gas revealed the oxygen insertion into dimer backbond sites. These facts indicate that a reaction path of the oxygen insertion into dimer backbonds through bridge sites is open for the clean surface oxidation, and the direct chemisorption probability at the backbonds is negligibly small comparing with that at the bridge sites.