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.

The interaction of low-energy XeF 2 with Si(100)(2 × 1) has been studied and compared to that of F 2 . Helium atom diffraction, beam-surface scattering, and thermal desorption measurements are the major techniques used in this study. It is found that XeF 2 dissociatively chemisorbs with high probability solely on the Si dangling bonds up to a coverage of about one monolayer (ML). Molecular fluorine has previously been observed to react similarly, saturating the dangling bonds at 1 ML coverage. The thermal desorption kinetics and products from the fluorinated layer produced by XeF 2 exposure are identical to those produced by F 2 exposure. The interactions of XeF 2 and F 2 are also strikingly similar with respect to the long-range order of the fluorinated Si up to about 1 ML coverage. The order is monitored by He diffraction. In both systems, the diffracted He beams exhibit a sharp decrease in intensity because of the disorder produced by the fluorination of random surface-unit cells as the coverage increases from 0 to about 0.3 ML. The intensity then increases until the fluorine overlayer has fully recovered its (2 × 1) periodicity at about 1 ML. This recovery corresponds to the decoration of each Si dangling bond with a fluorine atom. A critical observation of this study is that despite the large exothermicity of the dissociative chemisorption of XeF 2 or F 2 the order of the surface is not destroyed in either system. After saturation of the dangling bonds, F 2 ceases to react with the surface whereas XeF 2 continues to deposit fluorine by reacting with the Si-Si σ dimer bonds and the Si-Si lattice bonds. The order is destroyed as a result of the continued fluorine deposition, and ultimately, etching occurs by the formation of volatile SiF 4 . † Part of the special issue "John C. Tully Festschrift".

Xenon difluoride reacts with Si(100)2x1 by single atom abstraction whereby a dangling bond abstracts a F atom from XeF(2), scattering the complementary XeF product molecule into the gas phase, as observed in a molecular beam surface scattering experiment. Partitioning of the available reaction energy produces sufficient rovibrational excitation in XeF for dissociation of most of the XeF to occur. The resulting F and Xe atoms are shown to arise from the dissociation of gas phase XeF by demonstrating that the angle-resolved velocity distributions of F, Xe, and XeF conserve momentum, energy, and mass. Dissociation occurs within 2 A of the surface and within a vibrational period of the excited XeF molecule. Approximately an equal amount of the incident XeF(2) is observed to react by two atom abstraction, resulting in adsorption of a second F atom and scattering of a gas phase Xe atom. Two atom abstraction occurs for those XeF product molecules whose bond axes at the transition state are oriented within +/-60 degrees of the normal and with the F end pointed toward the surface.

The dissociative chemisorption of F 2 on the Si(100)(2 × 1) surface saturated with 1 monolayer (ML) of fluorine is investigated as a function of the incident F 2 translational energy. At energies below 3.8 kcal/mol, no reaction with the Si-Si bonds occurs. Above this threshold, the probability of dissociative chemisorption rises linearly with the normal component of the incident translational energy up to a value of 3.6 × 10 -3 at 13 kcal/mol. The relatively small effect of translational energy implies a late barrier in the potential energy surface for the interaction of F 2 with the Si-Si bonds. These probabilities are measured by exposing the fluorine-saturated surface to supersonic F 2 beams of variable energy, followed by thermal desorption measurements to determine the resulting fluorine coverage. Information regarding the specific Si-Si site (Si-Si dimer or Si-Si lattice bonds) at which the translationally activated reaction occurs is obtained from He diffraction measurements. The intensity of the diffracted beams is monitored after exposing the fluorinesaturated surface to F 2 of variable energy. The intensities remain constant after exposure to low-energy (<3.8 kcal/mol) F 2 , whereas they decline monotonically as a function of F 2 normal energy above the 3.8 kcal/mol threshold. Moreover, the similarity of the relative cross sections for diffusive scattering measured after exposure to translationally fast F 2 to those measured after Ar + ion bombardment strongly suggests that the reaction does not occur preferentially at the Si-Si dimer bonds, which are the weakest Si-Si bonds in the system. Reaction at Si-Si lattice bonds also occurs, leading to surface disorder. Additional data show that for submonolayer coverages generated from low energy F 2 , no reaction with Si-Si bonds occurs, while exposure to high-energy F 2 leads to reaction with Si-Si bonds.

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.