Reaction products in Si etching with a hot Cl2(Cl2*) molecular beam were measured by a quadrupole mass spectrometer. A major part of the product was shown to be SiCl4, and the reaction probability of Cl2*on a Si surface was obtained. It was found that the reaction probability increases rapidly with the increase in furnace temperature for Cl2*formation, and high reactivity of Cl2*was demonstrated. A reaction model (the activated complex Arrhenius model) is proposed to explain the experimental results, and the model parameters are determined. This model takes into account the effects of the translational and vibrational energies of a Cl2*molecule on the activation energy and the frequency factor in the ordinal Arrhenius model. ©1995 American Institute of Physics.