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Verbal Reinforcement of Self-Referent Affective Responses of Transitional Care Patients on a Modified Taffel Task

Description: The hypothesis of this study is: Transitional care patients reinforced for displaying a particular affective verbal habit (either positive or negative) opposite their original affective verbal habit (either negative or positive) will increase their tendency to display the reinforced affective verbal habit.
Date: August 1969
Creator: Lane, James R.
Partner: UNT Libraries

Habit-Forming Agents: Their Indiscriminate Sale and Use a Menace to the Public Welfare

Description: Report discussing the growth of drug addiction in the United States due chiefly to the consumption of drugs originally intended for medicinal purposes. Soothing syrups, soft drinks, medications for the nose and lungs, and remedies for headaches and epilepsy are the primary sources of habit-forming substances. In addition, medicines purporting to cure tobacco and drug addiction have, in many cases, also been found to contain addictive agents.
Date: 1910
Creator: Kebler, Lyman F. (Lyman Frederic), b. 1863
Partner: UNT Libraries Government Documents Department

Crystallography of the Delta to Alpha Martensitic Transformation in Plutonium Alloys

Description: A new stress-accommodating crystallographic mechanism of the {delta} {yields} {alpha} martensitic transformation in plutonium alloys is proposed. According to this mechanism, an orientation variant of the {alpha} phase is produced by a combination of a homogeneous strain and shuffling of the alternating close-packed (111){sub {delta}} planes. It is shown that the formation of stable transformation-induced twins whose twin plane orientations and twin shear directions do not depend on the small variations of the crystal lattice parameters is the preferred stress-accommodating mode. Only these stable twins have dislocation-free twin boundaries while the twin boundaries of all others are decorated by ultra-dense distribution of partial dislocations. The theory predicts a crystal lattice rearrangement mechanism involving the (205){sub {alpha}} ((01{bar 1}){sub {delta}}) stable twins. The corresponding Invariant Plane Strain solutions, with special emphasis on two simplest shuffling modes, the single and double elementary modes, are presented and compared with the existing experimental observations. It is shown that the habit plane orientation is highly sensitive to the input values of the crystal lattice parameters and especially to the accuracy of the measured volume change in the {delta}{yields}{alpha} transformation. An analysis of these effects on the habit plane orientation and orientation relations is also presented.
Date: July 15, 2004
Creator: Jin, Y; Wang, Y; Khachaturyan, A; Krenn, C & Schwartz, A
Partner: UNT Libraries Government Documents Department

Capacitance of edge plane of pyrolytic graphite in acetonitrile solutions

Description: The capacitance of the edge plane of pyrolytic graphite electrodes, in acetonitrile solutions, is measured by recording the current response to an applied triangular voltage sweep; TVS, and then fitting the current response with an appropriate function, (via a set of adjustable parameters). The pretreatment of the electrodes, the supporting electrolyte concentration used, and the frequency of the input TVS, were all found to affect the measured capacitance. In these experiments, a background current was also seen and the shape of the current output for the TVS; the charging/discharging curve, is shown to correlate with the magnitude of this background current. In addition, the size of the background current was found to have some dependence on the type of electrode pretreatment procedure used. 60 refs., 49 figs., 3 tabs.
Date: May 1, 1991
Creator: Minick, S. K. & Ishida, Takanobu
Partner: UNT Libraries Government Documents Department

Orientation relationship, plasticity, twin relationship, and interfacial structure of the ???' isothermal martensitic transformation in Pu-Ga alloys

Description: The orientation relationship, habit plane, parent-product interface at the atomic level, twin relationship, and plastic deformation resulting from the {delta} {yields} {alpha}{prime} isothermal martensitic transformation in Pu-Ga alloys are examined using optical microscopy, transmission electron microscopy, and finite element calculations. The {delta} {yields} {alpha}{prime} transformation exhibits a {approx}20% volume collapse when the face-centered cubic {delta} phase transforms to the monoclinic {alpha}{prime} phase, which results in unique and intriguing crystallography and morphology. Here, we show that the orientation relationship is very close to that previously reported by Zocco et al. (1990), but has small rotational misalignments between the two phases both parallel and perpendicular to the [110]{sub {delta}} {parallel}[100]{sub {alpha}{prime}} direction. The amount of plastic deformation is exceedingly large due to the {approx}20% volume collapse and transmission electron microscopy is used to quantify the difference in dislocation density between untransformed {delta}-matrix and regions of {delta} adjacent to the transformed {alpha}{prime}. The twins contained in {alpha}{prime} plates are shown to have a (205){sub {alpha}} orientation as the lattice invariant deformation and are found to be composed of two alternating variants that share a common <020>{sub {alpha}{prime}} direction, but differ by a 60 degree rotation about <020>{sub {alpha}{prime}}. A combination of electron diffraction and optical microscopy has been employed to examine the macroscopic habit plane and the analysis suggests that a large fraction of the observed habit planes are on or near {l_brace}111{r_brace}{sub {delta}}. Finally, high resolution transmission electron microscopy reveals that the interface is faceted on {l_brace}111{r_brace}{sub {delta}}, exhibiting a series of terrace and ledges.
Date: January 24, 2006
Creator: Moore, K; Krenn, C; Wall, M & Schwartz, A
Partner: UNT Libraries Government Documents Department

Characteristics of hydride precipitation and reorientation in spent-fuel cladding.

Description: The morphology, number density, orientation, distribution, and crystallographic aspects of Zr hydrides in Zircaloy fuel cladding play important roles in fuel performance during all phases before and after discharge from the reactor, i.e., during normal operation, transient and accident situations in the reactor, temporary storage in a dry cask, and permanent storage in a waste repository. In the past, partly because of experimental difficulties, hydriding behavior in irradiated fuel cladding has been investigated mostly by optical microscopy (OM). In the present study, fundamental metallurgical and crystallographic characteristics of hydride precipitation and reorientation were investigated on the microscopic level by combined techniques of OM and transmission electron and scanning electron microscopy (TEM and SEM) of spent-fuel claddings discharged from several boiling and pressurized water reactors (BWRs and PWRs). Defueled sections of standard and Zr-lined Zircaloy-2 fuel claddings, irradiated to fluences of {approx}3.3 x 10{sup 21} n cm{sup {minus}2} and {approx}9.2 x 10{sup 21} n cm{sup {minus}2} (E > 1 MeV), respectively, were obtained from spent fuel rods discharged from two BWRs. Sections of standard and low-tin Zircaloy-4 claddings, irradiated to fluences of {approx}4.4 x 10{sup 21} n cm{sup {minus}2}, {approx}5.9 x 10{sup 21} n cm{sup {minus}2}, and {approx}9.6 x 10{sup 21} n cm{sup {minus}2} (E > 1 MeV) in three PWRs, were also obtained. Microstructural characteristics of hydrides were analyzed in as-irradiated condition and after gas-pressurization-burst or expanding-mandrel tests at 292-325 C in Ar for some of the spent-fuel claddings. Analyses were also conducted of hydride habit plane, morphology, and reorientation characteristics on unirradiated Zircaloy-4 cladding that contained dense radial hydrides. Reoriented hydrides in the slowly cooled unirradiated cladding were produced by expanding-mandrel loading.
Date: November 14, 2000
Creator: Chung, H. M.; Strain, R. V. & Billone, M. C.
Partner: UNT Libraries Government Documents Department

Capacitance of edge plane of pyrolytic graphite in acetonitrile solutions

Description: The capacitance of the edge plane of pyrolytic graphite electrodes, in acetonitrile solutions, is measured by recording the current response to an applied triangular voltage sweep; TVS, and then fitting the current response with an appropriate function, (via a set of adjustable parameters). The pretreatment of the electrodes, the supporting electrolyte concentration used, and the frequency of the input TVS, were all found to affect the measured capacitance. In these experiments, a background current was also seen and the shape of the current output for the TVS; the charging/discharging curve, is shown to correlate with the magnitude of this background current. In addition, the size of the background current was found to have some dependence on the type of electrode pretreatment procedure used. 60 refs., 49 figs., 3 tabs.
Date: May 1, 1991
Creator: Minick, S.K. & Ishida, Takanobu.
Partner: UNT Libraries Government Documents Department

Mechanisms and controlling characteristics of the catalytic oxidation of methane. Progress report

Description: Progress has included (1) construction and installation of an ultraviolet photoelectron spectrometer (UPS) with power supply and pumping system that has been added as an attachment to the SCIENTA ESCA-300 instrument, (2) modification of the high resolution electron energy loss spectrometer (HREELS) to produce a stable ultra high vacuum (UHV) environment for initial experiments with a Pd(311) single crystal, (3) construction of a separate high vacuum system for preparation of surface doped model catalysts by chemical vapor deposition and pretreatment of a Pd(100) single crystal in this system, (4) carried out detailed experiments of methane activation and oxidation on Pd(679) using a high pressure reaction cell contained in a third ultra high vacuum system, (5) completion of adsorption/desorption studies of H{sub 2}, CO, and O{sub 2} on Pd(679), (6) utilized angle-resolved XPS to probe the diffraction characteristics and structure of the Pd(100) surface, (7) determination of the electronic surface structure of Pd(100) using angle-resolved UPS, and (8) computational analysis of oxygen overlayers on the PD(100) surface. Each of these is discussed in further detail below.
Date: December 31, 1990
Creator: Klier, K.
Partner: UNT Libraries Government Documents Department

Surface structure, bonding, and dynamics: The universality of zincblende (110) potential energy surfaces

Description: Using a tight-binding, total energy (TBTE) model we examine the hypothesis that the potential energy surfaces (PES) describing the (110) cleavage faces of the tetrahedrally coordinated, zincblende-structure compound semiconductors exhibit a common ``universal`` form if expressed in terms of suitably scaled parameters. TBTE calculations on both III-V and II-VI compounds reveal a linear scaling with bulk lattice constant of the geometric parameters of the reconstructed surfaces. This scaling is analogous to that found using low-energy, electron-diffraction surface-structure determination. The surface atomic force constants (found from a TBTE calculation) also scale monotonically with the lattice constant. Using TBTE models proposed previously for GaP, GaAs, GaSb, InP, and ZnSe, we find that the force constants scale as the inverse square of the bulk lattice constant. These results suggest that if distances are measured in units of the bulk lattice constant, the PES may be a universal function for the cleavage surface of zincblende-structure compound semiconductors, on average, with small fluctuations from this average occurring in individual materials. 22 refs., 5 figs., 1 tab.
Date: November 1, 1991
Creator: Godin, T. J. & LaFemina, J. P.
Partner: UNT Libraries Government Documents Department

[The activation and decomposition of alkanes on group VIII transition metal surfaces: Dynamics, kinetics and spectroscopy]. Progress report

Description: Significant progress has been achieved in a number of areas with the support of this Department of Energy grant which began on July 1, 1989. We have quantified experimentally the kinetics of the initial (low-coverage) alkane activation reaction, i.e., alkane {yields} alkyl + hydrogen adatom, for the following systems: (1) C{sub 2}H{sub 6} on Ir(110)-(1{times}2), and (2) CH{sub 4}, CD{sub 4}, C{sub 2}H{sub 6} and C{sub 2}D{sub 6} on Pt(110)-(1{times}2). In addition, we have employed dynamic Monte Carlo simulations to study two important aspects of surface reactivity, namely, reactant segregation during a steady-state surface reaction, and the compensation effect in a transient surface reaction. Experimentally determined values of the initial adsorption probability of ethane on Ir(110)(1{times}2) were employed to probe the dynamics of the interaction. Experimentally determined values of the initial trapping probability {zeta}{sub 0} of ethane into a physically adsorbed state at T{sub s} = 77 K as a function of E{sub i} and {Theta}{sub i} and experimentally determined values of the initial probability of dissociative chemisorption P{sub r} as a function of E{sub i}, {Theta}{sub i} and T{sub s} were obtained.
Date: December 31, 1990
Creator: Weinberg, W. H.
Partner: UNT Libraries Government Documents Department

[Selective carbon oxygen bond scission during reactions of oxygenates on single crystal catalysts]. Progress report

Description: We have discovered that the carbon-oxygen bond in methanol can be selectively broken if the surface structure of the platinum catalyst is appropriately tailored. The objective of this project is to determine if variations in surface structure allow one to selectively break C-O and C-H bonds. The decomposition of a wide range of oxygenates on several carefully chosen faces of group VIII metals will be examined to see when C-O bond scission occurs and what new chemistry we can find on stepped surfaces.
Date: August 1, 1992
Partner: UNT Libraries Government Documents Department

Investigation of ultrafast photothermal surface expansion and diffusivity in GaAs via laser-induced dynamic gratings

Description: This thesis details the first direct ultrafast measurements of the dynamic thermal expansion of a surface and the temperature dependent surface thermal diffusivity using a two-color reflection transient grating technique. Studies were performed on p-type, n-type, and undoped GaAs(100) samples over a wide range of temperatures. By utilizing a 90 fs ultraviolet probe with visible excitation beams, the effects of interband saturation and carrier dynamics become negligible; thus lattice expansion due to heating and subsequent contraction caused by cooling provided the dominant influence on the probe. At room temperature a rise due to thermal expansion was observed, corresponding to a maximum net displacement of {approximately} 1 {Angstrom} at 32 ps. The diffracted signal was composed of two components, thermal expansion of the surface and heat flow away from the surface, thus allowing a determination of the rate of expansion as well as the surface thermal diffusivity, D{sub S}. By varying the fringe spacing of the grating, this technique has the potential to separate the signal contributions to the expansion of the lattice in the perpendicular and parallel directions. In the data presented here a large fringe spacing was used, thus the dominant contribution to the rising edge of the signal was expansion perpendicular to the surface. Comparison of he results with a straightforward thermal model yields good agreement over a range of temperatures (20--300{degrees}K). Values for D{sub S} in GaAs were measured and found to be in reasonable agreement with bulk values above 50{degrees}K. Below 50{degrees}K, D{sub S} were determined to be up to an order of magnitude slower than the bulk diffusivity due to increased phonon boundary scattering. The applicability and advantages of the TG technique for studying photothermal and photoacoustic phenomena are discussed.
Date: April 1, 1992
Creator: Pennington, D. M.
Partner: UNT Libraries Government Documents Department

Image potential states at metal-dielectric interfaces

Description: Angle-resolved two-photon laser photoemission was used to observe the image potential electronic states on the (111) face of a silver single crystal. The transient image potential states were excited from the occupied bulk bands with photons whose energy was tunable around 4 eV. Photoemission of the image potential states was accomplished with photons of energy tunable around 2 eV. Image potential states were found to persist in the presence of physisorbed adlayers of xenon and cyclohexane. On clean Ag(111), the effective mass of the n=1 image potential state was found to be 1.4{plus_minus}0.1 times the mass of a free electron (m{sub e}). A binding energy of 0.77 eV, measured by earlier workers, was assumed in analysis of the data for the clean surface. On Ag(111), at 75 K covered by one monolayer of xenon, the binding energy of the n=1 image potential state was unchanged relative to its value on the clean surface. An effective mass of (1.00{plus_minus}0.05) {center_dot} m{sub e} was obtained. On Ag(111) at 167 K, covered by one monolayer of cyclohexane, the binding energy of the n=2 member of the image potential series was 0.30{plus_minus}0.05 eV. The energy of the n=1 state was again unchanged by deposition of the adsorbate. The effective masses of both states were (0.90{plus_minus}0.1) {center_dot} m{sub e}.
Date: April 1, 1992
Creator: Merry, W. R. Jr.
Partner: UNT Libraries Government Documents Department

Mechanisms and controlling characteristics of the catalytic oxidation of methane

Description: Progress has included (1) construction and installation of an ultraviolet photoelectron spectrometer (UPS) with power supply and pumping system that has been added as an attachment to the SCIENTA ESCA-300 instrument, (2) modification of the high resolution electron energy loss spectrometer (HREELS) to produce a stable ultra high vacuum (UHV) environment for initial experiments with a Pd(311) single crystal, (3) construction of a separate high vacuum system for preparation of surface doped model catalysts by chemical vapor deposition and pretreatment of a Pd(100) single crystal in this system, (4) carried out detailed experiments of methane activation and oxidation on Pd(679) using a high pressure reaction cell contained in a third ultra high vacuum system, (5) completion of adsorption/desorption studies of H{sub 2}, CO, and O{sub 2} on Pd(679), (6) utilized angle-resolved XPS to probe the diffraction characteristics and structure of the Pd(100) surface, (7) determination of the electronic surface structure of Pd(100) using angle-resolved UPS, and (8) computational analysis of oxygen overlayers on the PD(100) surface. Each of these is discussed in further detail below.
Date: January 1, 1990
Creator: Klier, K.
Partner: UNT Libraries Government Documents Department

(The activation and decomposition of alkanes on group VIII transition metal surfaces: Dynamics, kinetics and spectroscopy)

Description: Significant progress has been achieved in a number of areas with the support of this Department of Energy grant which began on July 1, 1989. We have quantified experimentally the kinetics of the initial (low-coverage) alkane activation reaction, i.e., alkane {yields} alkyl + hydrogen adatom, for the following systems: (1) C{sub 2}H{sub 6} on Ir(110)-(1{times}2), and (2) CH{sub 4}, CD{sub 4}, C{sub 2}H{sub 6} and C{sub 2}D{sub 6} on Pt(110)-(1{times}2). In addition, we have employed dynamic Monte Carlo simulations to study two important aspects of surface reactivity, namely, reactant segregation during a steady-state surface reaction, and the compensation effect in a transient surface reaction. Experimentally determined values of the initial adsorption probability of ethane on Ir(110)(1{times}2) were employed to probe the dynamics of the interaction. Experimentally determined values of the initial trapping probability {zeta}{sub 0} of ethane into a physically adsorbed state at T{sub s} = 77 K as a function of E{sub i} and {Theta}{sub i} and experimentally determined values of the initial probability of dissociative chemisorption P{sub r} as a function of E{sub i}, {Theta}{sub i} and T{sub s} were obtained.
Date: January 1, 1990
Creator: Weinberg, W.H.
Partner: UNT Libraries Government Documents Department

Electron stimulated desorption of the metallic substrate at monolayer coverage: Sensitive detection via 193 nm laser photoionization of neutral aluminum desorbed from CH{sub 3}O/Al(111)

Description: A fortuitous overlap between the gain profile of the 193 nm ArF excimer laser and the Al autoionizing transition {sup 2}S{sub 1/2} (512753 cm{sup {minus}1}) {l_arrow} {sup 2}P{sup 0}J has been exploited in the direct observation of substrate metal atoms in an electron simulated desorption (ESD) process from the monolayer adsorbate system CH{sub 3}O/Al(111). The identity of the mass 27 photoion was established as Al{sup +} by (1) isotopic substitution of {sup 13}C in the methanol employed for methoxy formation, and (2) tunable laser scans utilizing the {sup 2}DJ (J=3/2, 5/2) intermediate levels at {approximately}32436 cm{sup {minus}1} and a 248 nm ionization step. An ESD yield of {approximately}{times}10{sup {minus}6} Al atoms/(electron at 1 keV) was established by comparison with a sputtering experiment in the same apparatus. Velocity distributions measured for the desorbed Al species showed some differences in comparison with methoxy velocity data: a slightly lower peak velocity and a significantly less prominent high-velocity component. 10 refs., 4 figs.
Date: July 1, 1992
Creator: Young, C. E.; Whitten, J. E.; Pellin, M. J.; Gruen, D. M. & Jones, P. L.
Partner: UNT Libraries Government Documents Department

Catalysis at experimentally designed surfaces: n-Butane hydrogenolysis at Sn/Group VIII surface alloys

Description: Bimetallic {radical}3x{radical}3R30{degrees}-Sn/M(111) [M=Ni, Pt] surface alloys have been prepared in an ultrahigh vacuum system and used in model moderate pressure (1--200 Torr) catalytic reactions. Hydrogenolysis of n-butane (H{sub 2}/n-C4=20) has been used to characterize the effects of ordered bimetallic ensembles relative to those available at the respective (111) surfaces. The effect of alloying Sn into the Ni(111) surface produced an overall lowering of the catalytic reactivity nearly equal to the number of Ni surface sites lost upon producing the {radical}3x{radical}3R30{degrees} surface alloy. In addition, the overall rate of hydrogenolysis of n-butane at the {radical}3x{radical}R30{degrees}-Sn/Pt(111) surface alloy was also found to have decreased (although not in proportion to the loss of total Pt atoms). Catalytic deactivation due to carbon buildup was observed to decrease significantly at both of the ordered {radical}3x{radical}3R30{degrees}-Sn/M(111) surface alloys under reaction conditions.
Date: February 1, 1992
Creator: Logan, A. D. & Paffett, M. T.
Partner: UNT Libraries Government Documents Department

The dynamics of adsorption on clean and adsorbate-modified transition metal surfaces. [Progress report, 1989--1991]

Description: Research is being carried out on both nondissociative and dissociative adsorption of gases on transition metal surfaces, with emphasis on alkanes. Particular attention is be placed on understanding the effects of adsorbed species on adsorption probabilities and on clarifying the role of energy exchange processes at the surface in both dissociative and nondissociative adsorption. Molecular beam methods are coupled with methods of surface science to gain the greatest control of the variables of the problem, including the incident kinetic energy, vibrational energy and angle of incidence of the incoming molecules. The dynamics of both direct and precursor-influenced dissociative adsorption routes are examined. Surface intermediates formed by activated adsorption are identified by vibrational spectroscopy and temperature programmed reaction. Emphasis is on comparative studies of low molecular weight alkanes in order to understand the origin of the differences in their reactivities on different metals.
Date: June 1, 1991
Creator: Madix, R. J.
Partner: UNT Libraries Government Documents Department

The dynamics of adsorption on clean and adsorbate-modified transition metal surfaces

Description: Research is being carried out on both nondissociative and dissociative adsorption of gases on transition metal surfaces, with emphasis on alkanes. Particular attention is be placed on understanding the effects of adsorbed species on adsorption probabilities and on clarifying the role of energy exchange processes at the surface in both dissociative and nondissociative adsorption. Molecular beam methods are coupled with methods of surface science to gain the greatest control of the variables of the problem, including the incident kinetic energy, vibrational energy and angle of incidence of the incoming molecules. The dynamics of both direct and precursor-influenced dissociative adsorption routes are examined. Surface intermediates formed by activated adsorption are identified by vibrational spectroscopy and temperature programmed reaction. Emphasis is on comparative studies of low molecular weight alkanes in order to understand the origin of the differences in their reactivities on different metals.
Date: June 1, 1991
Creator: Madix, R.J.
Partner: UNT Libraries Government Documents Department

Electron stimulated desorption of the metallic substrate at monolayer coverage: Sensitive detection via 193 nm laser photoionization of neutral aluminum desorbed from CH sub 3 O/Al(111)

Description: A fortuitous overlap between the gain profile of the 193 nm ArF excimer laser and the Al autoionizing transition {sup 2}S{sub 1/2} (512753 cm{sup {minus}1}) {l arrow} {sup 2}P{sup 0}J has been exploited in the direct observation of substrate metal atoms in an electron simulated desorption (ESD) process from the monolayer adsorbate system CH{sub 3}O/Al(111). The identity of the mass 27 photoion was established as Al{sup +} by (1) isotopic substitution of {sup 13}C in the methanol employed for methoxy formation, and (2) tunable laser scans utilizing the {sup 2}DJ (J=3/2, 5/2) intermediate levels at {approximately}32436 cm{sup {minus}1} and a 248 nm ionization step. An ESD yield of {approximately}{times}10{sup {minus}6} Al atoms/(electron at 1 keV) was established by comparison with a sputtering experiment in the same apparatus. Velocity distributions measured for the desorbed Al species showed some differences in comparison with methoxy velocity data: a slightly lower peak velocity and a significantly less prominent high-velocity component. 10 refs., 4 figs.
Date: January 1, 1992
Creator: Young, C.E.; Whitten, J.E.; Pellin, M.J.; Gruen, D.M. (Argonne National Lab., IL (United States)) & Jones, P.L. (STI Optronics, Bellevue, WA (United States))
Partner: UNT Libraries Government Documents Department

Catalysis at experimentally designed surfaces: n-Butane hydrogenolysis at Sn/Group VIII surface alloys

Description: Bimetallic {radical}3x{radical}3R30{degrees}-Sn/M(111) (M=Ni, Pt) surface alloys have been prepared in an ultrahigh vacuum system and used in model moderate pressure (1--200 Torr) catalytic reactions. Hydrogenolysis of n-butane (H{sub 2}/n-C4=20) has been used to characterize the effects of ordered bimetallic ensembles relative to those available at the respective (111) surfaces. The effect of alloying Sn into the Ni(111) surface produced an overall lowering of the catalytic reactivity nearly equal to the number of Ni surface sites lost upon producing the {radical}3x{radical}3R30{degrees} surface alloy. In addition, the overall rate of hydrogenolysis of n-butane at the {radical}3x{radical}R30{degrees}-Sn/Pt(111) surface alloy was also found to have decreased (although not in proportion to the loss of total Pt atoms). Catalytic deactivation due to carbon buildup was observed to decrease significantly at both of the ordered {radical}3x{radical}3R30{degrees}-Sn/M(111) surface alloys under reaction conditions.
Date: January 1, 1992
Creator: Logan, A.D. & Paffett, M.T.
Partner: UNT Libraries Government Documents Department

Surface structure, bonding, and dynamics: The universality of zincblende (110) potential energy surfaces

Description: Using a tight-binding, total energy (TBTE) model we examine the hypothesis that the potential energy surfaces (PES) describing the (110) cleavage faces of the tetrahedrally coordinated, zincblende-structure compound semiconductors exhibit a common universal'' form if expressed in terms of suitably scaled parameters. TBTE calculations on both III-V and II-VI compounds reveal a linear scaling with bulk lattice constant of the geometric parameters of the reconstructed surfaces. This scaling is analogous to that found using low-energy, electron-diffraction surface-structure determination. The surface atomic force constants (found from a TBTE calculation) also scale monotonically with the lattice constant. Using TBTE models proposed previously for GaP, GaAs, GaSb, InP, and ZnSe, we find that the force constants scale as the inverse square of the bulk lattice constant. These results suggest that if distances are measured in units of the bulk lattice constant, the PES may be a universal function for the cleavage surface of zincblende-structure compound semiconductors, on average, with small fluctuations from this average occurring in individual materials. 22 refs., 5 figs., 1 tab.
Date: November 1, 1991
Creator: Godin, T.J. & LaFemina, J.P.
Partner: UNT Libraries Government Documents Department

Image Potential States at Metal-Dielectric Interfaces

Description: Angle-resolved two-photon laser photoemission was used to observe the image potential electronic states on the (111) face of a silver single crystal. The transient image potential states were excited from the occupied bulk bands with photons whose energy was tunable around 4 eV. Photoemission of the image potential states was accomplished with photons of energy tunable around 2 eV. Image potential states were found to persist in the presence of physisorbed adlayers of xenon and cyclohexane. On clean Ag(111), the effective mass of the n=1 image potential state was found to be 1.4{plus minus}0.1 times the mass of a free electron (m{sub e}). A binding energy of 0.77 eV, measured by earlier workers, was assumed in analysis of the data for the clean surface. On Ag(111), at 75 K covered by one monolayer of xenon, the binding energy of the n=1 image potential state was unchanged relative to its value on the clean surface. An effective mass of (1.00{plus minus}0.05) {center dot} m{sub e} was obtained. On Ag(111) at 167 K, covered by one monolayer of cyclohexane, the binding energy of the n=2 member of the image potential series was 0.30{plus minus}0.05 eV. The energy of the n=1 state was again unchanged by deposition of the adsorbate. The effective masses of both states were (0.90{plus minus}0.1) {center dot} m{sub e}.
Date: April 1992
Creator: Merry, W. R., Jr.
Partner: UNT Libraries Government Documents Department