Extrinsic States Research Articles

The dynamics of argon and methane trapping on Pt(111) at 30 and 50 K: energy scaling and coverage dependence

The dynamics of argon and methane trapping on clean Pt(111) were investigated using supersonic molecular beam techniques at a surface temperatures of 30 and 50 K, well below the desorption temperatures of 46 and 67 K, respectively. The initial trapping probabilities for both Ar and methane scale with normal incident energy ( E T cos 2 θ), indicating a smooth gas–surface potential. The trapping probability for Ar decreases from 0.8 to zero as the incident normal energy is increased from 2 to 29 kJ mol −1. Below 10 kJ mol −1 incident energy, the results agree with previous experiments [C.B. Mullins et al., Chem. Phys. Lett. 163 (1989) 111] and molecular dynamics simulations [M. Head-Gordon et al., J. Chem. Phys. 94 (1991) 1516] which were conducted above the desorption temperature. The trapping probability for methane decreases from 0.7 to zero as the incident normal energy is increased from 3 to 20 kJ mol −1. Trapping on the argon- or methane-saturated surface is greatly enhanced compared to the clean surface at all incident energies and angles, and exhibits near total energy scaling ( E T cos 0.3 θ), indicating a corrugated gas–surface potential. The kinetics of trapping as a function of coverage are quantitatively described by the modified Kisliuk model [H.C. Kang et al., J. Chem. Phys. 92 (1990) 1397; C.R. Arumainayagam et al., J. Chem. Phys. 94 (1991) 1516], which allows for an extrinsic precursor to adsorption. The trapping probabilities of both argon and methane increase with coverage, indicating that trapping into an extrinsic precursor state is more efficient than trapping onto the bare Pt(111) surface.

Chemisorption of NH3 on Si(100)-(2×1): A study by first-principles ab initio and density functional theory

The energetics involved in the chemisorption of NH3 on Si(100)-(2×1) have been examined using ab initio and nonlocal density functional theory. One- and two-dimer cluster models were employed to model the Si(100)-(2×1) surface. By using various exchange-correlation functionals and Gaussian split-valence basis sets, we have obtained the geometries of the molecularly adsorbed and dissociatively chemisorbed states, as well as the hitherto unreported geometry of the transition state which exists between the two states. The geometries of the various states have been rationalized based on either electrostatic or orbital interactions. In addition, calculations were also performed on models which contain a second-adlayer ammonia molecule to yield several possible geometries for the extrinsic precursor state. The extrinsic precursor ammonia binding energies for the various geometries found are in the range of 3.93–8.80 kcal/mol. The energetics of the chemisorption process and the binding energies of the extrinsic precursor ammonia are in good agreement with available experimental data.

Investigations of splitting of the π bands in graphite by scanning tunneling spectroscopy

The identification of features of the graphite electronic density of states is of great importance in scanning tunneling microscopy/spectroscopy studies of physically adsorbed molecules on this substrate. The results of scanning tunneling spectroscopy on the graphite surface enable us to resolve the Q 2g −(π), Q 2u −(π), Q 2u −(π*), Q 2g −(π*) states and estimate the π bands splitting. The measured splitting for the π* bands is 0.5 eV, while for the π bands 0.8 eV. The measured energy of the interlayer state stays within the range of 3.8–3.9 eV above the Fermi level, while the energy position of the extrinsic state varies from 2.5 eV to 2.9 eV above the Fermi level. The peaks detected near 5.5 eV below the Fermi level and 6 eV above the Fermi level are tentatively interpreted as the Γ 3g +(σ) and Γ 3u +(σ*) states, respectively.

Options and sunspots in a simple monetary economy

We study sunspot immunity in a dynamic monetary economy in which consumers are allowed to trade put and call option contracts on the general price level. We define the concept of strong sunspot immunity to characterize economies that have no sunspot equilibria regardless of the number of extrinsic states and their probabilities of occurrence. We show that a small number of option contracts can make an economy strongly sunspot immune. In addition, we demonstrate how asset re-trading opportunities, and the associated capital gains and losses, reduce the number of options needed for this result to obtain.

High pressures and the Kondo gap inCe3Bi4Pt3s

We have measured the electrical resistivity \ensuremath{\rho}(T) of single crystals of ${\mathrm{Ce}}_{3}$ ${\mathrm{Bi}}_{4}$ ${\mathrm{Pt}}_{3}$ for temperatures from 1.2 to 300 K, and pressures from 1 bar to 145 kbar. The transport is dominated at high temperatures by excitations across a small activation gap \ensuremath{\Delta}, which increases rapidly with pressure. The low-temperature transport involves variable range hopping among extrinsic states in the gap. The spatial extent of the in-gap states reflects coupling to conduction-electron states, and is strongly modified as pressure enhances \ensuremath{\Delta}. Despite the strong pressure dependence of \ensuremath{\Delta}, a direct correspondence between single-ion energetics and the measured gap is maintained, and the role of valence fluctuations is minimal even at the highest pressures.

Complex impedance analyses of n-BaTiO3 ceramics showing positive temperature coefficient of resistance

The influence was studied of grain boundary layer modifiers on the impedance spectra of semiconducting BaTiO3 ceramics processed in various atmospheres. Ceramic samples of undoped BaTiO3 annealed in an N2+H2 atmosphere showed a single semicircle in the Cole–Cole plot when ohmic Ni electrodes were used. Impedance spectra of donor-doped specimens sintered in air showed overlapping semicircles which may indicate relaxational processes within the grains, e.g. electron trapping and detrapping at the deep traps. This is strongly supported by the minima shown by the grain resistance (Rg) around the Curie point (Tc), whereas the behaviour of the grain boundary resistance (Rgb) with temperature is similar to the d.c. resistance, i.e. a positive temperature coefficient of resistance (PTCR). Electron paramagnetic resonance (EPR) results indicate that the charge trapping is caused by the redistribution of electrons at native and extrinsic acceptor states during phase transitions. Thus, the neutral barium vacancies (VXBa) present in the tetragonal and orthorhombic phases change into singly ionized barium vacancies (V′Ba) in the cubic and rhombohedral phases. From the correlation between the resistivity changes and the charge redistribution at the trap states, it is envisaged that the PTCR effect is related to charge trapping at the acceptor states having higher concentration in the regions of the grain boundary layer. The impedance spectral data support this conclusion.

Optical gain and laser emission in HgCdTe heterostructures

A detailed study of stimulated emission mechanisms as well as laser emission capability has been carried out on Hg1−xCdxTe (0.44<x<0.5) separate-confinement heterostructures grown by molecular beam epitaxy. At low temperature, spontaneous photoluminescence (PL) occurs on extrinsic levels below the gap whereas optical gain exhibits a maximum of stimulated emission shifted towards higher energy, close to the gap. As temperature increases, spontaneous PL is shifted from the extrinsic states to the band-to-band transition by a thermally activated detrapping of the carriers. Above 100 K, spontaneous and stimulated emission vary in a similar way with temperature. Laser emission has been observed up to room temperature for all the heterostructures. The use of quantum wells in the active layer and graded index in the barriers has allowed a significant reduction of the excitation density threshold, as compared to a single separate-confinement heterostructure (SCH) of same composition. However, the high-temperature characteristic temperature T0 is found to be similar in the two structures. A SCH with a higher energy gap exhibits a more favorable behavior with temperature. These experimental results have been compared to theoretical models. The experimentally observed T0 can be well simulated by taking into account the Auger effect. From the experimental data, the Auger constant has been determined for each heterostructure.

Theoretical and numerical studies of chemisorption on a line with precursor layer diffusion.

Models for surface adsorption with mobile precursor states have been studied since the work of Kisliuk to describe gas deposition on a metal surface. As the adsorbate coverage increases, these systems can develop nontrivial correlations that are induced by the diffusion mediated deposition mechanism. Our purpose in this paper is to investigate a model with precursors that is simple enough to be mathematically solved, at least approximately, without completely neglecting the positional correlations between the chemisorbed sites. We consider a model for random deposition of monomers on a line with extrinsic precursor states. In a numeric simulation, we study various quantities describing the evolution of the island structure. We write down the exact kinetic equations for this process and solve these equations within a simple, self-consistent theory that incorporates pair correlations. The results for the correlations, island density number, average island size, and probabilities of island nucleation, growth, and coagulation show good agremeent with the simulation data. (c) 1995 The American Physical Society

Correlated equilibrium and sunspot equilibrium

We show by an example that the sunspot equilibria of a competitive economy are not equivalent to the correlated equilibria if sunspots generate transfers between (extrinsic) states of nature (through a contingent commodities market). Nevertheless, we prove that the sunspot equilibrium allocations of a standard overlapping generations economy coincide with the (strategic form) correlated equilibrium allocations of a natural market game mimicking the economy.

Injection spectroscopy of localized states in thin insulating layers on semiconductor surfaces

An overview of charge injection techniques for studies of localized electron states in insulators is presented. These states control most important electrical properties of the materials, especially their thin layers and the multilayered structures containing them. Electronic properties of thin insulating layers are usually different from those of bulk material because in vicinity of an interface the thin film is subjected to action of structural strain, spatial confinement, extra energy release during growth or deposition, e.c. All these factors substantially affect the spectrum of localized electron states (intrinsic defects, dopants, impurities) and influence their spatial distribution. The interfaces often exhibit an unusual behaviour relative the defect generation and an impurity transport and segregation comparing with a bulk of insulator. The characterization of these states in thin-layered systems suffers from low sensitivity of both electron and optical spectroscopies to shallow electron states those have to be detected on the background of the signal from substrate. Nevertheless, the problem may be resolved by means of charge carrier injection into the insulating layer followed by monitoring of the charge becomes trapped. The localized states are detected by this way as a potential wells for the injected a mobile charge carriers. In thin layers the mean path of injected electron (hole) with respect to interaction with such a trap may greatly exceed thickness of the layer. In offers possibility to use linear modes for the trapping and detrapping analysis. We will show that under these conditions the charge-injection methods provide the most important phenomenological parameters of a localized state: the density per unit area, in-depth location, cross section for capture of carriers or recombination, energy position of the trapped carrier level in the insulator bandgap, e.c. On the base of these possibilities a non-destructive methods for characterization of extrinsic and intrinsic defects states are developed. Various charge injection and charge detection methods are compared. It shows unambiguously the advantages of using a semiconducting substrate for the insulator trapping studies. The limits of the linear models applicability are discussed in detail and the effects arising from the non-linear correlation phenomena are analyzed.

Dissociative chemisorption of oxygen on the Ru(001) surface: Spectroscopic identification of precursor intermediates at low surface temperatures

The adsorption of oxygen on the Ru(001) surface at 30 K has been investigated by high-resolution electron energy loss spectroscopy. Dissociative chemisorption occurs up to the saturation atomic coverage [ν⊥(Ru–O)=595 cm−1], after which a small amount of a molecularly chemisorbed peroxide species was isolated [ν(O–O)=790 cm−1]. After saturation of these atomic and molecularly chemisorbed states, physically adsorbed oxygen with a vibrational frequency close to the gas phase value of 1555 cm−1 was observed. The physically adsorbed oxygen was found to exhibit a resonance in the electron scattering cross section at an electron beam energy of about 5.5 eV. Heating the surface to 55 K leads to desorption of the physically adsorbed overlayer. The peroxide species, on the other hand, dissociates between 100 and 450 K. Although the initial precursors to dissociative chemisorption were not isolated, the peroxide identifies an intermediate precursor state, and the physically adsorbed oxygen identifies an extrinsic precursor state.

O 2 interaction with Pt{100}-hex-R0.7°: scattering, sticking and saturating

The interaction of oxygen with the Pt{100}-hex-R0.7° surface has been studied using supersonic molecular beams at incident translational energies from 0.06 to 0.9 eV and surface temperatures from 300 to 600 K. Scattering measurements show the existence of both intrinsic and extrinsic precursor states, and the trapping probability into these states is high at low incident energies. However, sticking probability measurements on the clean surface indicate that O 2 dissociative adsorption on Pt{100}-hex-R0.7° is a direct activated process, in contrast to that on Pt{100}-(1 × 1) or Pt{111}. Strong temperature enhancement of the initial sticking probability has been observed and accounted for partly by a dynamical barrier model. The sticking probability varies strongly with oxygen coverage, which is explained through computer simulations of island formation. The formation of small islands is demonstrated by TEAS measurements. Thermal desorption measurements show that, at high incident energies above 0.5 eV, new states are populated and higher coverages, up to a full monolayer, are reached.

The optical absorption and photoluminescence spectra of C60 single crystals

A constant photocurrent method (CPM) was used to determine the relative optical absorption coefficient of the C60 single crystal in the weak-absorption region. The result suggested the existence of extrinsic states in the C60 single crystal. The photoluminescence (PL) peaks of the C60 single crystal were observed at 1.48 eV and 1.65 eV these positions are different from those of C60 thin films.

Electronic properties of the HF-passivated Si(111) surface during the initial oxidation in air

The electronic properties (surface potential and surface state distribution) of the HF-treated Si(111) surface are investigated by the pulsed surface photovoltage technique during the initial oxidation in air. The HF-treated Si surface is characterized by positive surface charge leading to strong inversion for p-type Si and to accumulation for n-type Si independent of the HF treatment time. A large amount of extrinsic states below midgap is observed on the Si surface after long HF treatment times. During the initial oxidation in air the positive surface charge and the extrinsic states decrease and the electronic properties become more like those known for the thermally oxidized Si/SiO2 interface but generally with a higher density of states.

Dissociative chemisorption of O 2 on Cu(110)

The initial sticking coefficient S 0 for the dissociative chemisorption of O 2 on Cu(110) has been measured as a function of translational energy, surface temperature and scattering azimuth using a molecular beam. Sticking is facile on this face, having an initial sticking coefficient S 0 = 0.2 at 0.05 eV and rising to S 0 = 0.8 at translational energies above 0.35 eV. The dissociation probability is independent of scattering azimuth and follows a normal (cos 2 θ i) energy scaling. At low beam energies the sticking coefficient shows a strong surface temperature dependence, S 0 rising to ∽ 0.6 at 100 K. For beam energies above ∽ 0.2 eV the sticking coefficient is independent of surface temperature. This is interpreted in terms of two competing mechanisms, a trapping-desorption channel which dominates at low translational energies and low surface temperature ( T s) and a direct, activated dissociative chemisorption channel which becomes important as the energy is increased above 50 meV. The trapping state is thought to be a weakly bound, physisorbed molecular state, while the weak temperature dependence at high beam energies indicates direct, activated dissociation with negligible kinetic competition due to desorption via an (unstable) molecularly chemisorbed state. The coverage dependence S(θ) changes from a linear dependence at high beam energy and surface temperature to a slower decrease with θ at low T s, characteristic of an extrinsic precursor state. The trapping channel can be modelled as a function of θ and T s using a simple kinetic model for the precursors, based on the kinetic parameters obtained for the intrinsic precursor state and assuming local ordering of the adsorbate, even at the lowest surface temperatures (100 K).

Metallic clusters surface interaction: The case of Pd/SiOx/n-Si(100)

Atomic Force Microscopy (A.F.M.) and Rutherford back scattering (R.B.S.) have been employed to study the nucleation and growth phenomena of Palladium clusters cattered onto a thin (20 A) layer of SiOx, grown by plasma oxidation onto Si(100) doped B surface. X ray photoemission (XPS) of the 3d core level of palladium has permitted to follow the clusters silica interaction. Intrinsic final state size effect and extrinsic initial chemical state are discussed for clusters in the range of 10 to 104 atoms.

The Value of Money in an Overlapping Generations Model: A Note

In this paper a simple overlapping generations model—with N perishable commodities, no intrinsic uncertainty, one agent per generation with an intertemporally separable utility function, and one asset; money—is constructed. To this basic model, two extrinsic states of nature ( h ∈ {1, 2}) are added, at date 2. Sufficient conditions are found to ensure a continuum of rational expectations equilibria exist with the property that money is worthless if h = 1. The result is then extended to money becoming worthless with positive probability, for a finite number of dates. Journal of Economic Literature Classification Numbers: C62, D51, E41.

A theoretical study of the parameters affecting the kinetics of gas adsorption on solid surfaces

The nondissociative adsorption of gas phase molecules onto a spatially homogeneous square lattice was studied using a Monte Carlo computer simulation method which accounts for the existence of an extrinsic precursor state. The parameters affecting the adsorption kinetics were varied systematically in order to compare our simulations with a model for adsorption originally proposed by Kisliuk [J. Phys. Chem. Solids 3, 95 (1957)]. Our studies indicate that in the presence of precursor mobility, the sticking coefficients obtained from our simulations are consistently lower than those obtained using Kisliuk’s equation. Upon further investigation, we found two reasons for this discrepancy, namely, islanding of the adsorbates on the surface, and site revisiting in the precursor state. We found that some degree of islanding within the chemisorption layer occurs as a direct consequence of the nature of the precursor mediated adsorption process and that the extent of this clustering is controlled by the competition between mobility within the physisorbed layer and desorption; when mobility dominates, islanding formation is enhanced. Site revisiting within the precursor state was also found to affect the adsorption kinetics; we were able to derive here an analytical expression for the sticking coefficient that accounts for this factor and that correlates well with results from the corresponding simulation. Finally, we show that sticking coefficients are affected by surface geometry.

Hydrogen films at low temperatures

Hydrogen (deuterium) adsorption has been studied by the static skin effect method for W(110)H 2, D 2 and Mo(110)H 2, D 2 metal-hydrogen systems in the temperature range of 1.7–30 K. The findings indicate stabilization of mobile molecular complexes, so-called extrinsic precursor states, as well as their probable participation in the formation of two-dimensional ordered structures, Eden clusters.

The structure of the set of sunspot equilibria in economies with incomplete financial markets

This paper considers the set of equilibria of two-period, sunspot economies withS purely extrinsic states of nature in the second period andI assets with linearly independent nominal payoffs. The span of the payoff matrix contains the vector [1, ... , 1] (i.e., inside money). The set of economies is described in terms of (sunspot-invariant) utility functions. IfS>I> 0, there is an open, dense set of economies such that, given a vector of no arbitrage asset prices, the set of equilibrium allocations contains a smooth manifold of dimensionS—I. Such a manifold contains at least one nonsunspot equilibrium (and at most a finite number of such equilibria).