In the interaction of low energy F 2 with Si͑100͒ at 250 K, a dissociative chemisorption mechanism called atom abstraction is identified in which only one of the F atoms is adsorbed while the other F atom is scattered into the gas phase. The dynamics of atom abstraction are characterized via time-of-flight measurements of the scattered F atoms. The F atoms are translationally hyperthermal but only carry a small fraction ͑ϳ3%͒ of the tremendous exothermicity of the reaction. The angular distribution of F atoms is unusually broad for the product of an exothermic reaction. These results suggest an ''attractive'' interaction potential between F 2 and the Si dangling bond with a transition state that is not constrained geometrically. These results are in disagreement with the results of theoretical investigations implying that the available potential energy surfaces are inadequate to describe the dynamics of this gas-surface interaction. In addition to single atom abstraction, two atom adsorption, a mechanism analogous to classic dissociative chemisorption in which both F atoms are adsorbed onto the surface, is also observed. The absolute probability of the three scattering channels ͑single atom abstraction, two atom adsorption, and unreactive scattering͒ for an incident F 2 are determined as a function of F 2 exposure. The fluorine coverage is determined by integrating the reaction probabilities over F 2 exposure, and the reaction probabilities are recast as a function of fluorine coverage. Two atom adsorption is the dominant channel ͓ P 2 ϭ0.83 Ϯ0.03(95%, Nϭ9)͔ in the limit of zero coverage and decays monotonically to zero. Single atom abstraction is the minor channel (P 1 ϭ0.13Ϯ0.03) at low coverage but increases to a maximum (P 1 ϭ0.35Ϯ0.08) at about 0.5 monolayer ͑ML͒ coverage before decaying to zero. The reaction ceases at 0.94Ϯ0.11(95%, Nϭ9) ML. Thermal desorption and helium diffraction confirm that the dangling bonds are the abstraction and adsorption sites. No Si lattice bonds are broken, in contrast to speculation by other investigators that the reaction exothermicity causes lattice disorder.

A model is developed to describe the kinetics of the three scattering channels-unreactive scattering and dissociative chemisorption via single atom abstraction and two atom adsorption-that are present in the interaction of F 2 with Si͑100͒. The model provides a good description of the non-Langmuirian coverage dependence of the probabilities of single atom abstraction and two atom adsorption, yielding insight into the dynamics of the gas-surface interaction. The statistical model is based on the premise that the two dissociative chemisorption channels share a common initial step, F atom abstraction. The subsequent interaction, if any, of the complementary F atom with the surface determines if the overall result is single atom abstraction or two atom adsorption. The results are consistent with the orientation of the incident F 2 molecular axis with respect to the surface affecting the probability of single atom abstraction relative to two atom adsorption. A perpendicular approach favors single atom abstraction because the complementary F atom cannot interact with the surface, whereas a parallel approach allows the F atom to interact with the surface and adsorb. The fate of the complementary F atom is dependent on the occupancy of the site with which it interacts. The model distinguishes between four types of dangling bond sites on the Si͑100͒͑2ϫ1͒ surface, based on the occupancy of the site itself and that of the complementary Si atom in the Si surface dimer. The results show that the unoccupied dangling bond sites on half-filled dimers are about twice as reactive as those on empty dimers, which is consistent with an enhanced reactivity due to a loss of a stabilizing interaction between the two unoccupied dangling bonds on a dimer.