Showing papers on "Saturation (graph theory) published in 1992"

Observation of nonsequential double ionization of helium with optical tunneling.

Abstract: We have measured the ion yields for helium ionized by 120 fs, 614 nm laser pulses for intensities near ${10}^{16}$ W/${\mathrm{cm}}^{2}$. We have found that for these ultrashort pulses the ${\mathrm{He}}^{+2}$ data exhibit a feature which saturates in parallel with the ${\mathrm{He}}^{+}$ signal indicating that the ionization may proceed nonsequentially. We propose a new mechanism, which can exist only in the tunneling regime, for such nonsequential ionization.

Calculations of high-order harmonic-generation processes in xenon at 1064 nm

Abstract: We perform detailed calculations of harmonic conversion in a 15-Torr jet of xenon into which a 1064-nm-wavelength 36-ps-pulse-width laser has been tightly focused, so that the peak intensity ranges from 5\ifmmode\times\else\texttimes\fi{}${10}^{12}$ to 5\ifmmode\times\else\texttimes\fi{}${10}^{13}$ W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The single-atom emission rates are obtained by integrating the time-dependent Schr\"odinger equation. We employ an improved atomic model which includes excitation and ionization through states with both low-lying ionic cores. The propagation equations are solved using a general and efficient finite-difference technique. Excellent agreement with experimental data is obtained. We consider the effect of the defocusing of the pump beam by the free electrons in the saturation regime, and find it to be small for the conditions studied here. The spatial, temporal, and spectral profiles of the harmonic emission are presented. Significant blueshifts of the harmonics' spectral line shapes are observed in the saturation regime. Finally, using model polarizations, we discuss how the harmonic fields build up in the nonlinear medium, through a series of interferences. It allows us to understand why efficient phase matching can be achieved in a strong-field regime for the laser-atom interaction.

Saturation of shadowing at very low Bjorken x.

Abstract: The ratio of cross sections for inelastic muon scattering on xenon and deuterium nuclei was measured at very low Bjorken {ital x} (0.000 02{lt}{ital x}{sub Bj}{lt}0.25). The data were taken at Fermilab experiment E-665 with a 490 GeV/{ital c} muon beam incident on liquid deuterium and gaseous xenon targets. Two largely independent analysis techniques gave statistically consistent results. The xenon-to-deterium per-nucleon cross-section ratio is constant at approximately 0.7 for {ital x}{sub Bj} below 0.003.

Optimized structures and electronic properties of alkali-metal (Na, K) -adsorbed Si(001) surfaces.

Abstract: The Na and K adsorption on the Si(001)-2\ifmmode\times\else\texttimes\fi{}1 surface is studied by first-principles molecular dynamics based on the norm-conserving pseudopotential. The stable adsorption sites are determined for full coverage (\ensuremath{\Theta}=1.0) and half coverage (\ensuremath{\Theta}=0.5). At \ensuremath{\Theta}=0.5, the configuration with Na and K adsorbed along the trough is more stable than predicted by the Levine model. The coverage dependence of the absorption energy suggests that the saturation coverage should be \ensuremath{\Theta}=1.0 for both Na and K and that a qualitative difference between Na and K can be observed in the growth mode of the adsorbed layer. Supplementary calculations for \ensuremath{\Theta}=1/3 and 5/6 are performed in order to obtain an insight into the difference between Na and K in the coverage dependence of the adsorption energy. It is pointed out that the partial core correction for the pseudopotentials of Na and K is very crucial in the structural stability arguments.

Metallic and insulating phases of Li x C 60 , Na x C 60 , and Rb x C 60

Abstract: Photoemission studies of Li and Na fullerides show a band of alkali-metal-induced states that is fully below the Fermi level for a stoichiometry of ${\mathit{A}}_{2}$${\mathrm{C}}_{60}$ and inverse photoemission results show a splitting of the unoccupied bands. Equivalent results for the Rb fullerides show the formation of only a metallic phase, ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$. For all three fullerides, doping to saturation produces nonmetallic ${\mathit{A}}_{6}$${\mathrm{C}}_{60}$ phases.

Trap-limited hydrogen diffusion in a-Si:H.

Abstract: Hydrogen transport in a-Si:H was investigated by deuterium diffusion experiments where the deuterium source was either a remote atomic deuterium plasma or a deuterated amorphous silicon layer. An enhanced hydrogen diffusion and a considerably lower diffusion activation energy (0.5 eV instead of 1.2--1.5 eV) is observed for diffusion from a plasma. This behavior is attributed to saturation of hydrogen configurations with high binding energy, which act as deep hydrogen traps and control hydrogen transport in diffusion experiments from a deuterated layer, by deuterium atoms injected from the plasma. Diffusion from a plasma is then dominated by hopping through states with low binding energy. A density-of-states distribution is derived for the hydrogen configurations controlling diffusion consisting of (i) a shallow state where hydrogen is weakly bonded and can diffuse with a small activation energy of 0.5 eV and (ii) deep states with binding energy larger than 1.2 eV and a total density of \ensuremath{\sim}${10}^{22}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. In glow discharge a-Si:H, hydrogen is bonded to deep states and only a relatively small fraction (20--60 %) of these states are empty.

Electron-energy-loss study of the oxidation of polycrystalline tin

Abstract: An electron-energy-loss (EELS) study has been carried out on polycrystalline Sn before and after room-temperature exposures of 100, 500, 1500, and 3500 L [1 Langmuir (L)=${10}^{\mathrm{\ensuremath{-}}6}$ Torr s] to ${\mathrm{O}}_{2}$ at low pressure (${10}^{\mathrm{\ensuremath{-}}6}$ Torr) and to ${\mathrm{O}}_{2}$ at high pressures (160 Torr for 5 min and air at 1 atm for 5 min). Depth-sensitive information was obtained from these surfaces by varying the primary-electron-beam energy from 100 to 600 eV and using inelastic mean-free-path calculations. The spectra have been interpreted based on features in the EELS spectra obtained from standard reference materials; Sn metal, SnO, and ${\mathrm{SnO}}_{2}$. During the 100-L exposure, ${\mathrm{O}}_{2}$ adsorbs dissociatively and forms SnO in the near-surface region. Subsurface SnO forms more deeply beneath the surface during the 500-L exposure, and a subsurface transitional oxide structure with a composition between that of SnO and ${\mathrm{SnO}}_{2}$ also forms. Higher exposures up to the EELS saturation exposure of 3500 L converts some of this transitional phase into subsurface ${\mathrm{SnO}}_{2}$. Angle-resolved EELS shows that the very near-surface region (outermost two or three atomic layers) consists almost entirely of SnO with Sn metal, transitional oxide, and ${\mathrm{SnO}}_{2}$ lying beneath the surface after a low-pressure, saturation exposure to ${\mathrm{O}}_{2}$. After a high-pressure exposure, the near-surface region is fully oxidized to a mixture of SnO and ${\mathrm{SnO}}_{2}$ with no metallic Sn. The ${\mathrm{SnO}}_{2}$ concentration is maximum at about 1.4 nm beneath the surface, and both SnO and transitional oxide are present throughout the 3.0-nm oxidized layer in varying quantities. The presence of moisture appears to accelerate the oxidation process in some undetermined manner.

Thermoluminescence characteristics of the 375 °C electron trap in quartz

Abstract: The thermoluminescence (TL) glow peak in quartz, which appears in the uv at a temperature of 375 \ifmmode^\circ\else\textdegree\fi{}C when a ramp heating rate of 20 \ifmmode^\circ\else\textdegree\fi{}C/s is used, was measured for emission in the uv as well as in the green part of the spectrum. Two glow peaks appear separated by about 20 \ifmmode^\circ\else\textdegree\fi{}C. Isothermal decay rates for both spectral components were measured at temperatures near the glow peaks. Heating ramp rates were varied to obtain data for Hoogenstraaten analyses. Additive dose curves up to saturation were obtained. Solar-simulator bleaching studies were performed. Overall, these data appear to suggest a kinetic order between first and second order. It was found possible by computer simulation to reproduce faithfully the observed data by use of a simple interactive model that possesses two separate recombination centers, both simultaneously fed by electrons from a single electron trap and transported through the conduction band. It was found necessary to attribute a distribution of activation energies to the electron trap, to allow for a significant retrapping probability, and to also invoke the presence of a thermally disconnected electron trap. The analysis comparing the theoretical predictions to the data used a single set of program parameters in obtaining computer-generated solutions to 100 coupled differential-integral equations.These parameters successfully generated the observed initial rise values ranging from 0.9 to 1.1 eV (depending on the method employed) and an observed Hoogenstraaten-analysis energy of 1.49 eV both with a distribution central activation energy of 1.45 eV. A glow curve asymmetry shape factor \ensuremath{\mu}=0.46 given by the theory matched the data well, suggesting an apparent intermediate kinetic order. The theory also simultaneously generated the required uv-TL and green-TL glow peak separation in temperature. A model for generating the necessary electron and hole densities during laboratory additive dosing to reproduce the observed dose growth data was developed. This model is a consistent extension of the prior theoretical analyses. The electron activation energy distribution also provides a ready explanation for the midterm fading phenomenon present when radiation doses acquired during long burial times are compared to laboratory administered doses required to produce the same TL signal.

Atomic structure, adsorbate ordering, and mode of growth of the K/Si(100)2 x 1 surface.

Abstract: We present an investigation of the structural properties, atomic structure, adsorbate mode of growth, and interface formation of the room-temperature K/Si(100)2\ifmmode\times\else\texttimes\fi{}1 system by a combination of core-level and valence-band photoemission spectroscopies using synchrotron-radiation and scanning-tunneling-microscopy experiments. The results indicate that, at saturation coverage, the potassium atoms appear to form one-dimensional chains parallel to the silicon dimer rows along the 〈110〉 direction 7.68 \AA{} apart with a single site of adsorption. These K chains pass over the surface step edges to be connected between themselves. The growth and occurrence of a second layer are clearly related to the presence of impurities which, even at very low levels, are shown to significantly increase the K sticking coefficient. It demonstrates the extreme sensitivity of this system and stresses the crucial importance of quality in surface preparation and alkali-metal deposition. At a lower coverage, the K atoms are adsorbed on various coexisting sites with no long-range order. An ordering transition leading to the formation of the one-dimensional linear K metallic chains occurs. The adsorbate-adsorbate interaction is the dominant driving force in this adsorbate-ordering transition. The valence-band results indicate that the K atoms are covalently bonded to the Si atoms through the dangling bond, which makes the cave the most favorable adsorption site for the K atoms. This investigation brings new insights into the understanding of the structural properties of (alkali metal)/Si(100)2\ifmmode\times\else\texttimes\fi{}1 systems.

New intrinsic defect in as-grown thermal SiO2 on (111)Si.

Abstract: K-band electron-spin resonance (ESR) has revealed an isotropic signal of g=2.002 46\ifmmode\pm\else\textpm\fi{}0.000 03 in as-prepared ${\mathrm{SiO}}_{2}$ thermally grown on (111)Si in dry ${\mathrm{O}}_{2}$ at 700\char21{}860\ifmmode^\circ\else\textdegree\fi{}C. The spectrum comprises a symmetric central signal of peak-to-peak width \ensuremath{\Delta}${\mathrm{\ensuremath{\beta}}}_{\mathrm{pp}}$=1.0 G amid a $^{29}\mathrm{Si}$ hyperfine doublet of splitting ${\mathit{a}}_{\mathrm{hf}}$=16.1 G. The salient ESR features point to an intrinsic defect in ${\mathrm{SiO}}_{2}$ characterized by an unpaired spin occupying an effectively nearly pure s state, which is not primarily localized at a Si site while exchanging a $^{29}\mathrm{Si}$ superhyperfine interaction with three equivalent neighboring Si sites.

Excitation-energy dependence of the giant dipole resonance width.

Abstract: High-energy \ensuremath{\gamma} rays have been measured in coincidence with heavy fragents in deeply inelastic reactions of $^{136}\mathrm{Xe}$${+}^{48}$Ti at 18.5 MeV/nucleon. The giant dipole resonance (GDR) strength function is deduced from an analysis of the photon spectra within the statistical model. The GDR width \ensuremath{\Gamma} is studied as a function of the fragment excitation energy ${\mathit{E}}^{\mathrm{*}}$. A saturation at about \ensuremath{\Gamma}=10 MeV is observed for ${\mathit{E}}^{\mathrm{*}}$/A\ensuremath{\ge}1.0 MeV/nucleon.

Smectic-C order, in-plane domains, and nematic reentrance in a microscopic model of liquid crystals.

Abstract: A microscopic model of liquid crystals, with frustrated dipolar, van der Waals, and benzene-ring steric hindrance interactions, yields reentrant phase diagrams with the sequence nematic--smectic-${\mathit{A}}_{\mathit{d}--}$nematic--smectic-${\mathit{A}}_{1--}$smectic-C. The Lindemann criterion of melting is adapted to liquid crystals and Monte Carlo simulation is used. The smectic-${\mathit{A}}_{1}$ and -C phases occur in two versions, one pointing to in-plane domain formation. Layer tilting is found to be due to permeation-rotation lock-in. The model yields three types of smectic-C phases distinguished by tilt saturation.

Experimental observation of enhanced nonresonant nonlinear optical responses from optically pumped electronic excited states

Abstract: Enhancement of greater than 2 orders of magnitude of nonresonant third-harmonic generation from an optically pumped ${\mathit{S}}_{1}$ electronic excited state is experimentally observed for quasi-two-dimensional conjugated disklike structures. The isotropically averaged ${\mathit{S}}_{1}$ excited-state microscopic susceptibility 〈${\ensuremath{\gamma}}_{1}^{\mathit{S}}$(-3\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega})〉 is -1640\ifmmode\pm\else\textpm\fi{}100\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}36}$ esu, and the corresponding macroscopic ${\mathrm{\ensuremath{\chi}}}_{\mathit{S}1}^{(3)}$(-3\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega}) exhibits temporal decay and pump-power saturation behavior associated with the expected decay and saturation of the ${\mathit{S}}_{1}$ state. The enhancement mechanism through optical pumping followed by a nonresonant nonlinear optical probe is generalizable to both second- and third-order optical processes in nonlinear optical media.

Stimulated Cherenkov emission in dielectric-lined waveguides

Abstract: Particle-in-cell simulations of a dielectric Cherenkov maser experiment are used to analyze the emissive characteristics of an annular relativistic beam in a dielectric-lined waveguide. Good agreement with recent experimental results is obtained. The large gain in the device follows linear theory until saturation, when particle bunching and trapping is observed. The beam density becomes highly modulated with pulse length \ensuremath{\sim}\ensuremath{\lambda}/2, where \ensuremath{\lambda} is the guide wavelength. The performance of the device in the high-current regime is highly nonlinear. A simple nonlinear theory, confirmed by numerical results, shows that the field saturates at a value corresponding to ${\mathrm{\ensuremath{\omega}}}_{\mathit{b}}$=\ensuremath{\Delta}\ensuremath{\omega}/2, where ${\mathrm{\ensuremath{\omega}}}_{\mathit{b}}$ is the electron bounce frequency in the wave and \ensuremath{\Delta}\ensuremath{\omega} is the detuning parameter.

Strong inhibition effect on secondary-electron emission induced by fast hydrogen clusters.

Abstract: We report on measurement of secondary-electron emission (SEE) induced by hydrogen clusters (H{sub {ital n}}{sup +}, {ital n}{le}19, odd) from thin carbon foils for beam velocities above and around the Bohr velocity. The {ital n} dependence of the SEE yield shows a strong inhibition effect with respect to the atomic case. At a given velocity, the inhibition effect is observed first to increase with {ital n} and then to reach a saturation value ({ital n}{ge}7). These results are connected to the understanding of the inhibition effects observed with molecular beams.

Upper Bounds on the Lightest Higgs Boson Mass in General Supersymmetric Standard Models

Abstract: In a general supersymmetric standard model there is an upper bound $m_h$ on the tree level mass of the $CP=+1$ lightest Higgs boson which depends on the electroweak scale, $\tan \beta$ and the gauge and Yukawa couplings of the theory. When radiative corrections are included, the allowed region in the $(m_h,m_t)$ plane depends on the scale $\Lambda$, below which the theory remains perturbative, and the supersymmetry breaking scale $\Lambda_s$, that we fix to $1\ TeV$. In the minimal model with $\Lambda=10^{16}\ GeV$: $m_h<130\ GeV$ and $m_t<185\ GeV$. In non-minimal models with an arbitrary number of gauge singlets and $\Lambda=10^{16}\ GeV$: $m_h<145\ GeV$ and $m_t<185\ GeV$. We also consider supersymmetric standard models with arbitrary Higgs sectors. For models whose couplings saturate the scale $\Lambda=10^{16}\ GeV$ we find $m_h<155\ GeV$ and $m_t<190\ GeV$. As one pushes the saturation scale $\Lambda$ down to $\Lambda_s$, the bounds on $m_h$ and $m_t$ increase. For instance, in models with $\Lambda=10\ TeV$, the upper bounds for $m_h$ and $m_t$ go to $415\ GeV$ and $385\ GeV$, respectively.

Interference effects in the degenerate-wave-mixing spectroscopy of alexandrite.

Abstract: The degenerate-multiwave-mixing spectroscopy of alexandrite at 77 K using a pulsed dye laser in resonance with the ${\mathit{R}}_{1\mathit{m}}$ line was studied in this work. Unusual asymmetric line shapes that can be explained by an interference effect between resonant and nonresonant contributions of the media susceptibility were observed. The resonant effect is caused by the saturation of the two-level system susceptibility and the nonresonant contribution comes from the linear polarizability difference between excited and ground states. The present work shows an observation of this kind of interference phenomena in degenerate-wave-mixing experiments. Furthermore, we note that it is an observation of saturation line splitting in degenerate wave mixing in a narrow line in solids.

Possibility of Phase Transitions Inducing Cold Fusion in Palladium/Deuterium Systems

Abstract: In this paper a tentative theory is presented in which {beta}-phase PdD{sub x} containing supersaturated deuterium transits into {beta}-phase PdD{sub x} containing less deuterium and {alpha}-phase PdD{sub x}. High-pressure ({approx}10 GPa) deuterium bubbles form at the same time. As the bubbles release energy, cracks are created in the PdD{sub x} crystal, and charge separation of deuterium occurs. Thus would cold fusion be induced. This proposal supports the fracture mechanism for cold fusion.

Spectral anomalies of even-mass Hg isotopes.

Abstract: The $^{184--200}\mathrm{Hg}$ isotopes are studied in the interacting-boson-approximation-plus-fermion-pair model. In general, energy levels up to I=22 can be reproduced quite well. In the low excitation energy region both the shape evolution and the (${8}^{+}$,${10}^{+}$,${12}^{+}$) triplet structure can be reproduced. The (${8}^{+}$,${10}^{+}$,${12}^{+}$) triplet structure can be understood as the saturation of isospin 1 fermion-pair interaction at the high spin end. The termination of triplet structure at I=12 strongly supports the (${\mathit{i}}_{13/2}$${)}^{2}$ band crossing. Striking agreements between the observed energy gaps of (${14}_{1}^{+}$,${12}_{1}^{+}$) and (${2}_{1}^{+}$,${0}_{1}^{+}$) states are considered as an indication of weak coupling between the boson core and the fermion pair. The B(E2,I\ensuremath{\rightarrow}I-2) are calculated and the spin dependence of the B(E2) values can be reproduced qualitatively. However, the detailed mass dependence is not correctly reproduced.

A Study on the Aquic Conditions of Two Natraqualfs in the Coastal Plain of Louisiana.

Abstract: 1. SOIL AQUIC CONDITIONS AND AQUIC MOISTURE REGIMES: A LITERATURE REVIEW ......... 1 1.

Dielectric permittivity of diacetylene single crystals in the chain direction and its dependence on the polymer content during solid-state polymerization in experiment and theory.

Abstract: The increase of the electric permittivity in the chain direction, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{r},\mathrm{?}}$, during in situ solid-state polymerization of diacetylene crystals depends linearly on the polymer content within experimental error. This linearity is understood to be independent of the distribution of chain lengths due to a saturation effect of the linear polarizability in the chain direction. The increase of ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{r},\mathrm{?}}$ by typically 1.8 from the monomer to the polymer value can be explained within a simple theory which takes into account a modified Genkin-Mednis approach for the monomer. Furthermore, the linearity is fulfilled for diacetylenes which show no structural phase transition during the polymerization process. By measuring ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{r},\mathrm{?}}$ the conversion curve can be determined with a very low consumption of substance and time compared with the point-to-point gravimetrical analysis.

Chemical probes of metal cluster structure--Fe, Co, Ni, and Cu

Abstract: Chemical reactivity is one of the few methods currently available for investigating the geometrical structure of isolated transition metal clusters. In this paper we summarize what is currently known about the structures of clusters of four transition metals, Fe, Co, Ni, and Cu, in the size range from 13 to 180 atoms. Chemical probes used to determine structural information include reactions with H{sub 2}(D{sub 2}), H{sub 2}0, NH{sub 3} and N{sub 2}. Measurements at both low coverage and at saturation are discussed.

Chemisorption and Thermal Decomposition of Benzene on Pd(110): High- Resolution Electron Energy Loss Spectroscopy, Low-Energy Electron Diffraction, and Thermal Desorption Studies.

Abstract: The adsorbed state of benzene on Pd(110) at 300 K and its thermal decomposition process in the temperature region up to 700 K have been investigated by using high-resolution electron energy loss spectroscopy, low-energy electron diffraction, and multiplexed thermal desorption spectroscopy. Vibrational spectra show the existence of two adsorbed states of benzene on Pd(110) at 300 K. In one state, benzene is adsorbed with its C ring nearly parallel to the surface (flat benzene), and in the other state, at some angle (tilted benzene). The tilted benzene is located in the c(4{times}2) domains. For a small exposure ({approx lt}0.3 langmuir; fractional coverage {theta}{sub C{sub 6}H{sub 6}}{approx lt}0.07), the flat benzene predominates. With increasing exposure, conversion from flat to tilted benzene occurs, and the amount of tilted benzene is increased relative to that of flat benzene. For the saturation exposure (3 langmuirs: {theta}{sub C{sub 6}H{sub 6}} {approximately} 0.27), the tilted benzene predominates; the flat benzene also exists. Thermal decomposition of benzene on Pd(110) has been studied in detail. For a small exposure (0.3 langmuir), heating to 380-600 K forms the C{sub x}H{sub y}(x {ge} 1, y = 0,1) species as the decomposition products. By heating the sample to 600 K, theymore » find only C adatoms exist on Pd(110). For the saturation exposure (3 langmuirs), by heating the sample to 400 K, they find the C(4{times}2) structure is well developed and the Pd(110) surface is mostly covered by tilted benzene. Heating to 400-650 K forms the C{sub x}H{sub y} (x = 1 or {ge} 3, y = 0,1) species. The decomposition temperature is shifted toward higher temperatures by the site-blocking effect of benzene admolecules themselves. The decomposition is accompanied by h{sub 2} desorption.« less

On the Eckhaus and the Benjamin-Feir Instabilty in the Vicinity of a Tricritical Point

Abstract: For pattern forming systems with spatial variations in one direction the Eckhaus instability, the instability against spatial modulations of the pattern, has been studied for a stationary forward bifurcation for a number of systems theoretically1–5 and experimentally6,7 and a characteristic band of wavelengths, which are linearly stable against spatial modulations, emerges in all these cases. Here we perform the analogous analysis at the tricritical point, at which the coefficient of the cubic term in the envelope equation vanishes and where we assume saturation to quintic order. It is found that the band of Eckhaus stable wavelengths is a factor of \( {{(1.5)}^{{\tfrac{1}{2}}}} \) wider in this case. We also summarize the results obtained for the Eckhaus instability recently in the case of a weakly inverted stationary bifurcation8, where the envelope equation takes the form $$ \dot{A} = \in A + \gamma {{A}_{{xx}}} + \beta |A{{|}^{2}}A - \delta |A{{|}^{4}}A $$ (1) and where ϵ, β, γ, and δ are real with β > 0 for a weakly inverted bifurcation.