Spontaneous desorption of Cl, Br, and I from n-and p-type Si͑100͒-͑2 ϫ 1͒ was studied with scanning tunneling microscopy at temperatures of 620-800 K where conventional thermal bond breaking should be negligible. The activation energies and prefactors determined from Arrhenius plots indicate a novel reaction pathway that is initiated by the capture of electrons which have been excited by phonon processes into Si-halogen antibonding states. This configuration is on a repulsive potential energy surface, and it is sufficiently long lived that desorption can occur, constituting phonon-activated electron-stimulated desorption. Surprisingly, the Arrhenius plots for differently doped samples crossed and, above a critical temperature, the reaction with the largest activation energy had the highest rate. This is explained by large entropy changes associated with the multiphonon nature of the electronic excitation. For Cl desorption from p-type Si, these entropy changes amounted to 34k B . They were 19k B , 13k B , and 8k B for Br desorption from p-type, lightly doped n-type, and heavily doped n-type Si, respectively. The desorption rates for I were nearly three orders of magnitude larger than the rates observed for Cl and Br. Here, the Si-I antibonding states overlap the conduction-band minimum, so that conduction-band electrons with this energy can be captured by the Si-I antibonding states. Together, these results reveal that a complex relationship exists between phonons and electronic excitations during chemical reactions at surfaces.

We investigated surface patterning of I-Si͑100͒-͑2 ϫ 1͒ both experimentally and theoretically. Using scanning tunneling microscopy, we first examined I destabilization of Si͑100͒-͑2 ϫ 1͒ at near saturation. Dimer vacancies formed on the terraces at 600 K, and they grew into lines that were perpendicular to the dimer rows, termed vacancy line defects. These patterns were distinctive from those induced by Cl and Br under similar conditions since the latter formed atom and dimer vacancy lines that were parallel to the dimer rows. Using first-principles density functional theory, we determined the steric repulsive interactions associated with iodine and showed how the observed defect patterns were related to these interactions. Concentration-dependent studies showed that the vacancy patterns were sensitive to the I concentration. Dimer and atom vacancy lines that were elongated along the dimer row direction were favored at lower coverage. Atom vacancy lines dominated at ϳ0.8 ML, they coexisted with dimer vacancy lines at ϳ0.6-0.7 ML, and dimer vacancy lines were exclusively observed below ϳ0.5 ML. These surface patterns reflect the mean strength of the adatom repulsive interactions.

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Lattice mismatch of Cu on Ag(111) produces fast diffusion for special "magic sizes" of islands. A sizeand shape-dependent reptation mechanism is responsible for low diffusion barriers. Initiating the reptation mechanism requires a suitable island shape, a property not considered in previous studies of 1D island chains and 2D closed-shell islands. Shape determines the dominant diffusion mechanism and leads to multiple clearly identifiable magic-size trends for diffusion depending on the number of atoms whose bonds are shortened during diffusion. PACS numbers: 68.35.Fx,68.35.bd,68.35.Gy,71.15.Pd 1 Control of thin-film morphology relies on understanding multiple ongoing processes during deposition and growth. In particular, diffusion of small atom clusters on surfaces play a critical role in thin film growth, especially in early stages. The diffusion kinetics of small islands in heteroepitaxial systems is less well understood than that of homoepitaxial diffusion, for which much experimental [1,2,3,4] and theoretical [5,6,7,8] work has been done. Strain is known to govern the mesoscale morphology in self-assembling systems [9]. While predictions about the role of size and misfit for small islands go back over a decade [10], only recent experiments have captured and quantified the rapid diffusion at "magic sizes" in the heteroepitaxial Cu/Ag (111)

Scanning tunneling microscopy combined with molecular dynamics simulations reveal a dislocation-mediated island diffusion mechanism for Cu on Ag(111), a highly mismatched system. Cluster motion is tracked with atomic precision at multiple temperatures and diffusion barriers and prefactors are determined from direct measurements of hop rates. The nonmonotonic size dependence of the diffusion barrier is in good agreement with simulations and can lead to enhanced mass transport upon coarsening, in surprising contrast to the traditional island diffusion models where diffusivity reduces with cluster size.