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We have investigated the surface chemistry of coadsorbed hydrogen and halogen atoms on Ge(100), produced by dissociative chemisorption of HCl and HBr, by temperature-programmed desorption. The initial sticking probability S0 for HCl decreases from 0.6 at a substrate temperature of 270 K to 0.05 at 400 K, indicative of a precursor state to adsorption. For HBr S0 is constant at 0.7 over the same temperature range. A fraction f of adsorbed hydrogen atoms desorb associatively as H2 near 570 K, while the remaining (1-f) H atoms recombine with adsorbed halogen atoms and desorb as the hydrogen halide (HX) near 580–590 K. The activation energies for desorption of H2, HCl, and HBr are all approximately 40 kcal/mol. For both HCl and HBr f is 0.7 at low initial coverage and decreases slightly to 0.6 at saturation. The fraction f of adsorbed halogen atoms left on the surface following the competitive desorption of H2 and HX desorb as the dihalides GeCl2 and GeBr2 near 675 and 710 K, respectively. Desorption of H2, HCl, and HBr occurs with near-first-order kinetics, similar to the behavior of hydrogen adsorbed alone, which we attribute to preferential pairing induced by the π bond on unoccupied Ge dimers. We introduce and solve a generalized doubly occupied dimer model incorporating competitive pairing of H+H, H+X, and X+X on Ge dimers to explain the near-first-order kinetics. The model quantitatively accounts for both the desorption kinetics and the relative yields of H2 and HX with pairing energies of ≊3 kcal/mol. Implications of the present results for surface thermochemistry, chemical vapor deposition, and atomic layer epitaxy of Ge and Si(100)2×1 surfaces are discussed. [ABSTRACT FROM AUTHOR]

Evidence is presented that π-bonding is present in the surface dimers on clean Si(100)–(2×1) and, as a consequence, that hydrogen atoms preferentially pair on surface dimer atoms even at low coverage. We propose a simple lattice gas model in order to calculate the equilibrium distribution of doubly- and singly-occupied dimers as a function of coverage and temperature, and show that even a very conservative estimate of the enthalpy difference between hydrogen on doubly- and singly-occupied dimers can explain the observed first-order desorption kinetics. [ABSTRACT FROM AUTHOR]

We show by temperature-programmed desorption that hydrogen desorbs from Ge(100)2×1 near 570 K with near-first-order kinetics, similar to the behavior of hydrogen on Si(100)2×1. The near-first-order desorption kinetics are attributed to pairing on surface dimers induced by the π bond on unoccupied dimer atoms, and a pairing energy of 5±1 kcal/mol is inferred. However, a comparison between the pairing energies for H atoms on Ge(100) and Si(100) with the electronic structure of the respective clean surfaces indicates that estimates of the π bond strength based on the surface band structure do not correlate with the propensity for pairing. [ABSTRACT FROM AUTHOR]

Oxygen exchange at a highly oriented La[sub 0.5]Sr[sub 0.5]CoO[sub 3-δ] thin film prepared on (100) surfaces of an yttria-stabilized zirconia single crystal by pulsed-laser deposition was studied with ac impedance spectroscopy under various temperatures and oxygen partial pressures. Three distinctive features observed in the impedance spectra were assigned to contributions from the ionic conduction of the electrolyte, oxide ion transfer across the electrode/electrolyte interface, and the oxygen exchange on the film surface. An equivalent circuit model was proposed to analyze the impedance results, from which the surface chemical exchange coefficients, k[sub chem], were derived as a function of temperature and oxygen partial pressure. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]