Etching reaction of Si(100) is investigated by usingCl2molecular beams with a hyperthermal translational energy up to 3.0 eV. The reaction rate is clearly enhanced by translational energy, and the threshold energy is 2.1 eV. The translational-energy-induced reaction rates are measured as a function of substrate temperature; the results closely fit Arrhenius plots as the sum of two components having activation energies of 2.7 and 1.2 eV. The higher energy, derived from the fit of the high-temperature region, agrees well with the pure thermal reaction, i.e., low-translational-energy reaction. The lower energy, which is observed here, is determined as the activation energy of the translational-energy-induced reaction. This energy is not affected by the translational energy from the threshold to 3.0 eV, whereas the etch rate increases with translational energy. These findings suggest that the translational energy contributes to the formation of a new chlorinated Si surface from which silicon chloride desorbs thermally with lower activation energy than from a chlorinated Si surface formed by a pure-thermal reaction. ©1997 American Institute of Physics.