Abstraction of a F atom from incident F2 by Si(100)-(2 x 1) is demonstrated by detection of the scattered, complementary F atom. He atom diffraction measurements are consistent with abstraction occurring at dangling bond sites. The low probability for single atom adsorption (P~= 0.10~0.01) relative to the total adsorption probability (P, = 0.96~0.02) in the zero coverage limit indicates that the second F atom can also be trapped by the dangling bonds. Both the single and two atom adsorption probabilities decay to zero when the dangling bonds are saturated at 1 ML coverage. Atom abstraction represents a mechanism distinct from the classic one for dissociative chemisorption. PACS numbers: 82.65.Pa, 34.50.Bw One important function of a surface in heterogeneous catalysis or chemical vapor deposition is to cleave a bond of an incident molecule, thus forming two adsorbed reactive fragments. The dissociation proceeds by the formation of two bonds to the surface because the formation of two bonds is energetically necessary for bond cleavage. This is the classic view of dissociative chemisorption [1], and indeed this mechanism is operative in many moleculesurface systems. In contrast, one can envision bond cleavage by formation of a single surface-atom bond if the energy released by this bond formation is greater than the bond energy of the incident molecule. Generically, this mechanism is known as abstraction and is a welldocumented reaction mechanism between two molecules in the gas phase. In this Letter, we document the dissociative chemisorption of a molecule on a surface by an abstraction mechanism.Specifically, we demonstrate that a Si (100)-(2 X 1) surface abstracts one F atom from an incident F2 molecule by detecting the complementary F atom scattered back into the gas phase with a triply differentially pumped, rotatable, quadrupole mass spectrometer in a molecular-beam -surface-scattering UHV apparatus. In addition, we couple these experiments with He atom diffraction from the resulting fluorinated surface to demonstrate that the Si surface dangling bonds are responsible for the abstraction and are the sites for F adsorption. Because of the experimental complexities in the detection of reactive radicals such as F atoms, this mechanism has gone undetected in numerous previous studies of the interaction of fluorine and fluorine containing molecules with Si. The apparatus has been described in detail [2]. The triply differentially pumped F2 beam is formed by expansion of a mixture of 1% F2 in Kr. The velocities of the incident and scattered beams are determined by cross correlation time-of-flight (TOF) methods. The beam is directed at an n-type Si(100) crystal that can be heated resistively and cooled to 120 K. The crystal is etched [3] prior to insertion into the vacuum chamber where it is repetitively sputtered and annealed (1120 K for 30 min) until Auger spectroscopy reveals carbon as the only contaminant, at a 1% or less level. He atom diffraction measurements show the surface to exhibit the welldocumented...

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.