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  • 1
    Electronic Resource
    Electronic Resource
    Modeling the Kinetics of Heterogeneous Catalysis (1995)
    s.l. : American Chemical Society
    Chemical reviews 95 (1995), S. 667-676 
    Details
    ISSN: 1520-6890
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/cr00035a010
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  • 2
    Electronic Resource
    Electronic Resource
    Initial probability of dissociative chemisorption of oxygen on iridium(110) (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 3440-3447 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociative chemisorption of oxygen on Ir(110) has been investigated using supersonic molecular beam techniques. The initial probability of dissociative chemisorption (in the limit of zero surface coverage) as a function of incident kinetic energy between 1 and 28 kcal/mol and surface temperature from 85 to 1000 K is reported. For beam kinetic energies less than approximately 4 kcal/mol, the measured values of the initial probability of dissociative chemisorption are explained by a trapping-mediated adsorption mechanism. In this adsorption regime initial probabilities of dissociative chemisorption decrease with both increasing beam energy and surface temperature. The trapping probability of oxygen into the physically adsorbed state on Ir(110) as a function of incident beam energy is presented. For beam kinetic energies greater than ∼4 kcal/mol, a direct chemisorption mechanism dominates. In the direct adsorption regime, initial probabilities of dissociative chemisorption increase with increasing beam energy, and they are dependent on surface temperature, with the dependence decreasing with increasing surface temperature. This behavior is attributed to direct chemisorption into a molecularly chemisorbed state, from which there is a thermally activated kinetic competition between desorption and dissociation. A pseudo-steady-state kinetic model including physically adsorbed oxygen, molecularly chemisorbed oxygen, and atomically chemisorbed oxygen is applied to find that the activation barrier to desorption from the physically adsorbed molecular state is 1.6±0.1 kcal/mol higher than the barrier to conversion to the molecularly chemisorbed state. The activation barrier for desorption from the molecularly chemisorbed state is 1.5±0.15 kcal/mol greater than the barrier to dissociation from this state. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469217
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  • 3
    Electronic Resource
    Electronic Resource
    Coadsorption of hydrogen with ethylene and acetylene on Si(100)-(2×1) (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 5605-5617 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The adsorption, desorption, and thermal decomposition of acetylene and ethylene on the Si(100)-(2×1) surface have been investigated with emphasis on the modifications induced by coadsorbed hydrogen. Based on high-resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption spectroscopy (TPD), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES), we show that the adsorption of acetylene and ethylene is blocked by preadsorbed hydrogen leading to a hydrocarbon saturation coverage which decreases linearly with hydrogen precoverage. At low temperatures preadsorbed hydrogen has no influence on the surface chemistry of acetylene or ethylene. At approximately 550 K, coadsorbed hydrogen induces decomposition of ethylene which is not observed in the absence of hydrogen. After postexposures of an ethylene-saturated Si(100)-(2×1) surface to gas-phase atomic hydrogen with fluences below 5×1014 cm−2, the ethylene is essentially unperturbed at low surface temperatures with partial decomposition upon heating as for preadsorbed hydrogen. Higher postexposures of atomic hydrogen lead to Si–C bond cleavage and the formation of ethyl. The desorption of molecular ethylene is then up shifted by approximately 100 K. The experimental results and observed reaction intermediate are explained by an elemental adsorption and reaction model. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472817
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  • 4
    Electronic Resource
    Electronic Resource
    Trapping-mediated dissociative chemisorption of C3H8 and C3D8 on Ir(110) (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 271-278 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have employed molecular beam techniques to investigate the molecular trapping and trapping-mediated dissociative chemisorption of C3H8 and C3D8 on Ir(110) at low beam translational energies, Ei≤5 kcal/mol, and surface temperatures, Ts, from 85 to 1200 K. For Ts=85 K, C3H8 is molecularly adsorbed on Ir(110) with a trapping probability, ξ, equal to 0.94 at Ei=1.6 kcal/mol and ξ=0.86 at Ei=5 kcal/mol. At Ei=1.9 kcal/mol and Ts=85 K, ξ of C3D8 is equal to 0.93. From 150 K to approximately 700 K, the initial probabilities of dissociative chemisorption of propane decrease with increasing Ts. For Ts from 700 to 1200 K, however, the initial probability of dissociative chemisorption maintains the essentially constant value of 0.16. These observations are explained within the context of a kinetic model which includes both C–H (C–D) and C–C bond cleavage. Below 450 K propane chemisorption on Ir(110) arises essentially solely from C–H (C–D) bond cleavage, an unactivated mechanism (with respect to a gas-phase energy zero) for this system, which accounts for the decrease in initial probabilities of chemisorption with increasing Ts. With increasing Ts, however, C–C bond cleavage, the activation energy of which is greater than the desorption energy of physically adsorbed propane, increasingly contributes to the measured probability of dissociative chemisorption. The activation energies, referenced to the bottom of the physically adsorbed molecular well, for C–H and C–C bond cleavage for C3H8 on Ir(110) are found to be Er,CH=5.3±0.3 kcal/mol and Er,CC=9.9±0.6 kcal/mol, respectively. The activation energies for C–D and C–C bond cleavage for C3D8 on Ir(110) are 6.3±0.3 kcal/mol and 10.5±0.6 kcal/mol, respectively. The desorption activation energy of propane from Ir(110) is approximately 9.5 kcal/mol. These activation energies are compared to activation energies determined recently for ethane and propane adsorption on Ir(111), Ru(001), and Pt(110)–(1×2), and ethane activation on Ir(110). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471872
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  • 5
    Electronic Resource
    Electronic Resource
    Interaction of gas-phase atomic deuterium with the Ru(001)–p(1×2)–O surface: Kinetics of hydroxyl and water formation (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 7713-7718 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The deuteration of oxygen adatoms on the Ru(001) surface has been investigated by means of temperature programmed desorption and high-resolution electron energy loss spectroscopy. Exposure of gas-phase atomic deuterium to the p(1×2) oxygen overlayer with a fractional adatom coverage of oxygen of 0.5 leads to the production of water at a surface temperature as low as 90 K. After exposure to molecular deuterium, no reaction is observed, suggesting that a direct Eley–Rideal (ER) reaction occurs between the impinging deuterium atoms and the preadsorbed oxygen. Only after a very low exposure of deuterium was it possible to isolate chemisorbed OD groups on the surface, implying that OD formation is the rate-limiting step in the formation of water via ER kinetics on Ru(001). Estimates of the ER reaction cross sections were made, and for the deuteration of adsorbed oxygen and hydroxyls, the cross sections were found to be (7.0±0.3)×10−17 cm2 and (2.2±0.1)×10−15 cm2, respectively. In addition to the ER mechanism, the chemisorbed OD groups could also react with coadsorbed deuterium adatoms via Langmuir–Hinshelwood (LH) kinetics at surface temperatures near 170 K, suggesting an activation barrier that is less than 9 kcal/mol. This implies that OD formation is also the rate-limiting step in the formation of water via LH kinetics on Ru(001). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471452
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  • 6
    Electronic Resource
    Electronic Resource
    Isotope effects in trapping-mediated chemisorption of ethane and propane on Ir(110) (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 3789-3793 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We compare here recent results of molecular beam investigations of the initial probability of trapping-mediated C–H and C–D bond cleavage of C2H6, C2D6, C3H8, and C3D8 on Ir(110) at low beam translational energy and surface temperatures, TS, from 85 to 800 K. Each of these systems is highly reactive at low TS and displays decreasing reactivity with increasing TS. Measurements of the initial probability of trapping-mediated chemisorption for both ethane and propane reveal an isotope effect, which we attribute to zero-point energy differences, with the perhydrido-species exhibiting greater reactivity at a given TS. A difference in activation energies for desorption vs reaction (C–D bond cleavage) for C2D6 has been found to be Ed–Er=1.8±0.3 kcal/mol, cf. Ed–Er=2.2 kcal/mol for C–H bond cleavage of C2H6. For the trapping-mediated dissociative chemisorption of propane on Ir(110), Ed–Er=4.2 kcal/mol for C–H bond cleavage of C3H8, and Ed–Er=3.2 kcal/mol for C–D bond cleavage of C3D8. A quantitative analysis of the initial probability of trapping-mediated dissociative chemisorption of ethane and propane on Ir(110), within the context of a classical kinetic model of barrier crossing from the physically adsorbed state to the dissociatively chemisorbed state, provides the most reasonable description of the observed adsorption behavior. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472199
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  • 7
    Electronic Resource
    Electronic Resource
    Direct dissociative chemisorption of propane on Ir(110) (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 11313-11318 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have employed molecular beam techniques to investigate the initial probability of direct dissociative chemisorption, Pd, and the intrinsic trapping probability, ξ, of C3H8, C3D8, and (CH3)2CD2 on Ir(110) as a function of beam translational energy, Ei, from 1.5 to 59 kcal/mol. For C3H8 and (CH3)2CD2, a measurable (≥ 0.02) initial probability of direct dissociative chemisorption is observed above a beam energy of approximately 7 kcal/mol. For C3D8 this energy is roughly 10 kcal/mol. Above these energies the initial probability of direct chemisorption of each of the isotopomers of propane increases nearly linearly with Ei, approaching a value of approximately Pd=0.48 at Ei=52 kcal/mol for C3H8 and (CH3)2CD2, and Pd=0.44 at Ei=59 kcal/mol for C3D8. This kinetic isotope effect for the direct chemisorption of C3D8 relative to C3H8 is smaller than that expected for a mechanism of H (or D) abstraction by tunneling through an Eckart barrier, suggesting a contribution of C–C bond cleavage to direct chemisorption. The lack of a kinetic isotope effect for the direct chemisorption of (CH3)2CD2 relative to C3H8 indicates that 1° C–H bond cleavage dominates over 2° C–H bond cleavage during the direct chemisorption of propane on Ir(110). The trapping behavior of each of these isotopomers of propane is approximately identical as a function of Ei, with ξ 〉0.9 at Ei=1.5 kcal/mol, ξ = 0.3 at Ei=20 kcal/mol, and ξ 〈 0.1 above Ei= 40 kcal/mol. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472871
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  • 8
    Electronic Resource
    Electronic Resource
    The dissociative chemisorption of cyclopropane on Ir(110) (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 7171-7176 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have employed molecular beam techniques to investigate the dissociative chemisorption of cyclopropane on Ir(110) as a function of beam translational energy, Ei, from 1.5 to 48 kcal/mol, and surface temperature, Ts, from 85 to 1200 K. For Ts=85 K, c-C3H6 is molecularly adsorbed on Ir(110) with a trapping probability, ξ, of 0.97 at Ei=1.5 kcal/mol and ξ=0.90 at Ei=5 kcal/mol. For Ei≤5 kcal/mol, c-C3H6 is dissociatively adsorbed through a mechanism of trapping-mediated chemisorption, with initial probabilities of chemisorption, Pa, decreasing with increasing surface temperature from the intrinsic trapping probability at Ts=150 K, to Pa〈0.05 above Ts=1000 K. The activation energy for trapping-mediated chemisorption of c-C3H6, referenced to the bottom of the physically adsorbed well and attributed to C–C bond cleavage, is 3.6±0.2 kcal/mol. For Ei≥10 kcal/mol, direct dissociative chemisorption increasingly contributes to the overall measured initial probability of chemisorption of cyclopropane. The initial probability of direct dissociative chemisorption of c-C3H6 increases approximately linearly from Pa=0.1 at Ei=10 kcal/mol, to Pa=0.5 at Ei=45 kcal/mol. No isotope effect is observed for the direct dissociative chemisorption of c-C3D6 for beam translational energies of 17 to 48 kcal/mol, indicating that C–C bond cleavage is the initial reaction coordinate for direct chemisorption of cyclopropane on Ir(110). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472519
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  • 9
    Electronic Resource
    Electronic Resource
    A two-dimensional ultrahigh vacuum positioner for scanning tunneling microscopy (1998)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 69 (1998), S. 1403-1405 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A two-dimensional ultrahigh vacuum compatible positioner is presented. The positioner uses two piezoelectric inchworms which allow for motions of up to 1 cm with a precision of 4 nm mounted at right angles to each other in order to give two dimensions of motion. Images of three-dimensional In0.3Ga0.7As islands in cross section are presented to demonstrate the functionality of the positioner. It is found that motion towards the tip is smooth, while motion in the perpendicular direction is less smooth. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1148773
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  • 10
    Electronic Resource
    Electronic Resource
    Effects of surface reconstruction on III–V semiconductor interface formation: The role of III/V composition (1999)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 74 (1999), S. 1704-1706 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Using molecular-beam epitaxy and in situ scanning tunneling microscopy, we demonstrate how different reconstructions associated with different III–V growth surfaces can create interfacial roughness, and that an understanding of this phenomenon can be used to control the roughness on the atomic scale. Specifically, the different compositions of a clean InAs(001)-(2×4) surface (V/III=0.5 ML/0.75 ML) and an Sb-terminated one (∼1.7 ML/1 ML) cause the InSb-like interfacial surface to have a bilevel morphology. This surface roughness can be eliminated by depositing additional In to exactly compensate for the difference. It is likely that similar types of roughness occur in all heterostructures where the growth surface reconstruction changes at the interfaces, and that a similar procedure will be equally effective at reducing that roughness. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.123661
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