bluuebunny/crossref_metadata_2025 huggingface dataset:
Intertidal colonization rates. A matched latitude, north v. south, remote v. near shore island experiment
Authors: DAVENPORT J. , STEVENSON T.
Published: December 2003

Colonization of artificial substrata was monitored on sheltered rocky shores at Husvik, South Georgia (54° 11′S; 36° 40′W) and Cumbrae, Scotland (55° 46′N; 4° 55′W) from mid summer to early autumn. South Georgia is a remote island (1,330 km from other land); Cumbrae is only 2 km from nearby coasts. Both islands were heavily glaciated for a period up to about 10,000 ybp, so the intertidal fauna is of geologically recent origin. The South Georgian fauna is depauperate and consists mainly of direct developers; that of Cumbrae is rich and largely larviparous. Colonization rates at Cumbrae were one to four orders of magnitude greater than at Husvik. It is suggested that the fauna of South Georgian shores stems from colonization by rafting from remote sources, while Cumbrae has been supplied predominantly by short‐range pelagic larval dispersal. The measured differences in colonization rates reflect the substantial local advantage of larval dispersal over direct development in established communities.
Fetal gender determination in early first trimester pregnancies of rhesus monkeys (Macaca mulatta) by fluorescent PCR analysis of maternal serum
Authors: Jimenez Daniel F. , Tarantal Alice F.
Published: November 2003

Abstract:  Non‐human primate fetal gender determination can be a powerful tool for research study design and colony management purposes. The recent discovery of the presence of fetal DNA in maternal serum has offered a new non‐invasive approach for identification of fetal gender. We present a rapid and simple method for the sexing of developing rhesus monkeys in the first trimester by polymerase chain reaction (PCR) analysis of maternal serum. Serum samples were obtained from 72 gravid rhesus monkeys during 20–32 days of gestation (term 165 ± 10 days). Fetal gender and the quantity of circulating fetal DNA were determined by real‐time PCR analysis of the rhesus Y‐chromosomal DNA sequences. The sensitivity for identifying a male fetus was 100% by 30 days gestation, and no false‐positive results were observed. This study demonstrates that fetal gender can be reliably determined in the early first trimester from maternal serum samples, a non‐invasive method for routine gender screening.
Between Xylem and Phloem: The Genetic Control of Cambial Activity in Plants
Authors: Helariutta Y. , Bhalerao R.
Published: November 2003

Abstract: Post‐embryonic development is controlled by two types of meristems: apical and lateral. There has been considerable progress recently in understanding the function of root and shoot apical meristems at the molecular level. Knowledge of analogous processes in the lateral, or secondary, meristems, i.e. the vascular cambium or cork cambium, is, however, rudimentary. This is despite the fact that much of the diversity in the plant kingdom is based on the differential functions of these meristems, emphasizing the importance of lateral meristems in the development of different plant forms. The vascular cambium is particularly important for woody plants, but it also plays an important role during the development of various herbaceous species, such as Arabidopsis thaliana. In this review, we focus on the two basic functions of cambial activity: cell proliferation and pattern formation.
Import Pathway of Nuclear‐Encoded Cytochrome c Oxidase Subunit 2 Using Yeast as a Model
Authors: Qualmann S. R. , Daley D. O. , Whelan J. , Pratje E.
Published: November 2003

Abstract: Subunit 2 of cytochrome c oxidase (Cox2) is a mitochondrial‐encoded protein in most organisms. In soybean Glycine max a second Cox2 gene was identified in the nucleus which is functional, whereas the mitochondrial‐encoded cox2 gene is silent. For import and sorting of the nuclear‐encoded soybean Cox2 protein (GmCox2p) into mitochondria, the protein has acquired an N‐terminal extension of 136 amino acid residues that is cleaved off in three steps during import. To study the function and processing of the GmCox2p leader peptide, we used yeast as a model system. Using different leader peptide‐GFP constructs, we were able to show that the i1 intermediate is generated in the mitochondrial matrix and the mature protein is generated in the inner membrane space. Mitochondrial processing peptidase (MPP) is involved in processing the first part of the leader peptide, processing of the last part is catalysed by the inner membrane peptidase (IMP). Oxa1p is necessary for insertion of the protein into the inner mitochondrial membrane. GmCox2p therefore utilises many of the same components as its mitochondrial‐encoded predecessor, for sorting and maturation, following its import into the mitochondria.
Compounds Interacting with the Ethylene Receptor in Plants
Authors: Sisler E. C. , Serek M.
Published: November 2003

Abstract: Some of the compounds binding to the ethylene receptor induce an ethylene response, but others prevent it. The compounds preventing an ethylene response have been developed into a means for protecting plants against ethylene and extending the life of some plant material. 1‐Methylcyclopropene (1‐MCP), a compound now commercially available under the names EthylBloc and SmartFresh™, is currently being used on flowers, fruit and vegetables with great success. In ethylene sensitive flowers, among other responses, it prevents senescence and abscission of plant organs; in fruit and vegetables it slows down the ripening process. Other similar compounds are now being developed for a range of methods of application.
Functional Gynodioecy in Opuntia quimilo (Cactaceae), a Tree Cactus Pollinated by Bees and Hummingbirds
Authors: Díaz L. , Cocucci A. A.
Published: November 2003

Abstract: The tree cactus Opuntia quimilo is one of three known gynodioecious cacti. Its flowers deviate from most Opuntias in features that are attributable to ornithophily: petals are shiny red in colour, and fleshy in consistency, a nectar chamber is present, and stamen seismonasty is lacking. Pollinators include large matinal bees (predominantly Ptilothrix tricolor and Megachile sp.) and hummingbirds (Chlorostilbon aureoventris and Heliomaster furcifer). Hummingbirds rarely visit other local Opuntias. Hummingbirds, which are more common in the afternoon, prefer female flowers whereas bees prefer hermaphroditic flowers. Female flowers have more dilute nectar than hermaphroditic flowers. Under experimental conditions female fertility is as high as that of hermaphrodites, however, seeds from females always result from cross‐pollination and from more severe ovule selection (ovule number is higher in female flowers). Under natural conditions female plants are reproductively more successful than hermaphrodites. Known cases of bird pollination in Opuntia and the incidence of ornithophilic features in Opuntia and related genera are discussed.
Flower Visitors in a Natural Population of Arabidopsis thaliana
Authors: Hoffmann M. H. , Bremer M. , Schneider K. , Burger F. , Stolle E. , Moritz G.
Published: November 2003

Abstract: Arabidopsis thaliana is commonly regarded as a self‐pollinated plant species. One of the many surprises in population genetic studies of the species was the observation of distinct traces of recombination in the DNA sequences that may be the result of rare outcrossing events. We studied flower visitors in a natural population of the species. Solitary bees, diptera and thrips are among the most frequently observed insects among the surprising diversity of insects visiting flowers of A. thaliana. Assuming that every visit equals an outcrossing event, the outcrossing rate was estimated to be 0.84 %. This value falls between estimations of outcrossing rates from molecular data and those of artificial systems. Despite the rather low rate of flower visitation, A. thaliana can no longer be regarded as a completely self‐pollinated plant species in the wild. This observation may explain recombination events observed in molecular analyses. Possible pollen transfer between populations due to the mobility of the observed insects should be considered in population genetic analyses.
Light and Electron Microscopic Study on the Mycorrhizae of Sporophytes of Botrychium virginianum ‐ Arbuscular Structure Resembling Fossil Forms
Authors: Kovács G. M. , Kottke I. , Oberwinkler F.
Published: November 2003

Abstract: The mycorrhizal structures of the sporophyte of the strictly protected fern Botrychium virginianum were studied by light and electron microscopy. The paper presented here is the first ultrastructural study of the mycorrhiza of the sporophyte of the genus Botrychium. The fungal colonization is restricted to two to three cell layers of the root cortex. The sometimes bifurcately ramifying hyphae form coils in the host cells. Single hypha arising from the coils penetrates into neighbouring cells and form arbuscules therein. The electron micrographs show high functional activity of the arbuscules, characterized by high glycogen‐like content in the hyphae, a thin layer of filamentous matrix material in the interface between fungus and root cell, frequent cell organelles in the cytoplasm and lack of starch in the host cells containing arbuscules. The main distinctive feature of the mycorrhizae of Botrychium virginianum is the morphology of the arbuscules, which differs not only from the common fine‐branched arbuscules, but also from the special structures reported previously from Ophioglossaceae. The robust, lobe‐like branching arbuscules most resemble arbuscule structures found in Triassic fossils.
Dandelion Seed Dispersal: The Horizontal Wind Speed Does Not Matter for Long‐Distance Dispersal ‐ it is Updraft!
Authors: Tackenberg O. , Poschlod P. , Kahmen S.
Published: November 2003

Abstract: Long‐distance dispersal of seeds (LDD) surely affects most ecological and evolutionary processes related to plant species. Hence, numerous attempts to quantify LDD have been made and, especially for wind dispersal, several simulation models have been developed. However, the mechanisms promoting LDD by wind still remain ambiguous and the effects of different weather conditions on LDD, although recognized as important, have only rarely been investigated. Here we examine the influence of wind speed and updrafts on dispersal of dandelion (Taraxacum officinale agg.), a typical wind‐dispersed herb of open habitats. We used PAPPUS, a weather‐sensitive mechanistic simulation model of wind dispersal, which considers frequency distribution of weather conditions during the period the simulation refers to. A simulation for the 4‐month shedding period of dandelion shows that high wind speed does not promote LDD. In contrast, vertical turbulence, especially convective updrafts, are of overwhelming importance. Mainly caused by updrafts, in the simulations more than 0.05 % of dandelion seeds were dispersed beyond 100 m, a distance commonly used to define LDD. We conclude that long‐distance dispersal of seeds of herbaceous species with falling velocities < 0.5 ‐ 1.0 ms‐1 is mainly caused by convective updrafts.
A Study of the Interaction between Auxin and Ethylene in Wild Type and Transgenic Ethylene‐Insensitive Tobacco during Adventitious Root Formation Induced by Stagnant Root Zone Conditions
Authors: McDonald M. P. , Visser E. J. W.
Published: November 2003

Abstract: Wild type (Wt) and transgenic plants (etr1‐1 gene from Arabidopsis thaliana; encoding for a defective ethylene receptor; Tetr) of Nicotiana tabacum L. were subjected to experiments to resolve the role of the interaction between ethylene and auxin in waterlogging‐induced adventitious root formation. Plants were grown in aerated or stagnant deoxygenated nutrient solution and treated with the following plant growth regulators: ethylene, the synthetic auxins 2,4‐dichlorophenoxyacetic acid (2,4‐D) and 1‐naphthaleneacetic acid (1‐NAA), and the auxin efflux inhibitor naphthylphthalamic acid (NPA). The superior growth of Wt in stagnant solution suggests that the ability to sense and respond to ethylene partially mediates tolerance to stagnant root zone conditions. Wt produced around 2 ‐ 2.5‐fold more adventitious roots than Tetr in aerated and stagnant solution. Treatment with NPA phenocopied the effects of ethylene insensitivity by reducing the number of adventitious roots on Wt to Tetr levels. Additionally, application of 1‐NAA to the shoot of Tetr increased the number of adventitious roots on Tetr to similar levels as the untreated Wt. However, this level was only around half the number achieved by 1‐NAA‐treated Wt. The results suggest an interplay between ethylene and auxin in the process of adventitious root formation in waterlogged tobacco, most likely on the level of polar auxin transport. However, a separate non‐auxin‐related role as a transcription regulator for genes essential to adventitious root formation cannot be excluded.
Antioxidant and Pigment Composition during Autumnal Leaf Senescence in Woody Deciduous Species Differing in their Ecological Traits
Authors: García‐Plazaola J. I. , Hernández A. , Becerril J. M.
Published: November 2003

Abstract: Photoprotection mechanisms have been studied during autumnal senescence in sun and shade leaves of woody plants with different ecological characteristics and senescence patterns. Three of them belonging to the same family, Betulaceae: the shade‐intolerant and early successional species (Betula alba L.), the shade‐tolerant and late successional species (Corylus avellana L.), and an N‐fixing tree with low N resorption efficiency (Alnus glutinosa L.). The other two species: a shade‐intolerant (Populus tremula L.) and a shade‐tolerant (Cornus sanguinea L.), were chosen because of their ability to accumulate anthocyanins during autumnal leaf senescence. The study of plants with different ecological strategies allowed us to establish general trends in photoprotection mechanisms during autumnal senescence, when nutrient remobilisation occurs, but also during whole leaf ontogeny. We have not found a clear relationship between shade tolerance and the level of photoprotection; the main difference between both groups of species being the presence of α‐carotene in shade leaves of shade‐tolerant species. Preceding autumn, nitrogen resorption started in mid‐summer and occurred in parallel with a slight and continuous ascorbate, chlorophyll and carotenoid degradation. However, the ascorbate pool remained highly reduced and lipid oxidation did not increase at this time. Contrasting with ascorbate, α‐tocopherol accumulated progressively in all species. Only during the last stages of senescence was chlorophyll preferentially degraded with respect to carotenoids, leading to the yellowing of leaves, except in A. glutinosa in which a large retention of chlorophyll and N took place. Senescing leaves were characterised, except in C. sanguinea, by a relative increase in the proportion of de‐epoxidised xanthophylls: zeaxanthin, antheraxanthin and lutein. The light‐induced accumulation of anthocyanins in C. sanguinea could play an additional protective role, compensating for the low retention of de‐epoxidised xanthophylls. These different strategies among deciduous species are consistent with a role for photoprotective compounds in enhancing nitrogen remobilization and storage for the next growing season.
Elevated pCO2 Affects C and N Metabolism in Wild Type and Transgenic Tobacco Exhibiting Altered C/N Balance in Metabolite Analysis
Authors: Kruse J. , Hetzger I. , Hänsch R. , Mendel R.‐R. , Rennenberg H.
Published: November 2003

Abstract: Diurnal changes in starch, sugar and amino acid concentrations in source leaves, sink leaves and roots of tobacco plants were determined. In addition to wild type tobacco, transformed plants deficient in root nitrate reductase and exhibiting decreased rates of growth were employed. Further, the growth rates of tobacco plants were modulated by exposure to elevated pCO2. From the diurnal alterations in metabolite concentrations, the daily turnover of starch and amino N was estimated in order to: (i) elucidate whether turnover rates can be related to growth rates, and (ii) identify individual amino compounds with the potential to indicate nitrogen fluxes and the C/N status of plants. Elevated pCO2 increased growth rates and daily turnover of starch in both wild type and transformed plants, indicating enhanced rates of photosynthesis. In wild type plants, elevated pCO2 increased the turnover of amino N, notably glutamine and alanine, in mature source leaves, indicating enhanced nitrate reduction. By contrast, amino N turnover in source leaves of transformed plants was not affected by elevated pCO2, although nitrate reduction was presumably enhanced. Apparently, export of amino N was increased from the source leaves of transformed plants. This assumption was supported by a significantly increased turnover of amino N in young sink leaves compared to mature source leaves, indicating a preference for acropetal amino N allocation and import into the young leaves of the transformed plants. Further, elevated pCO2 increased the allocation of leaf‐derived amino N to the roots of transformed plants. This led to increased levels of amino compounds during the entire day, notably glutamate, but did not affect root growth of the transformed plants. The suitability of individual amino compounds as markers for major N fluxes, such as nitrate reduction, photorespiration, and amino N export and import is discussed.
Evidence for the Polyphyly of Haworthia (Asphodelaceae Subfamily Alooideae; Asparagales) Inferred from Nucleotide Sequences of rbcL, matK, ITS1 and Genomic Fingerprinting with ISSR‐PCR
Authors: Treutlein J. , Smith G. F. , van Wyk B.‐E. , Wink M.
Published: November 2003

Abstract: Four molecular markers have been studied to examine the phylogenetic position of the South African plant genus Haworthia Duval within the succulent Asphodelaceae. Sequence data of the chloroplast genes matK and rbcL were compared to the nuclear markers ITS1 and ISSR (Inter Simple Sequence Repeat) analysis. Both lines of molecular data, chloroplast and nuclear DNA, indicate that Haworthia is polyphyletic, forming two distinct clades. Most taxa previously combined as Haworthia subgenus Haworthia branch off early in the alooid chloroplast trees forming a strongly monophyletic group, whereas subgenus Hexangulares forms a polyphyletic assemblage comprising other alooid genera. The nuclear markers ITS1 and ISSR fingerprinting support the two groups as distinctly different, therefore confirming the division seen in chloroplast DNA. The practical implication is that the generic concept of Haworthia may have to be restricted to H. subgenus Haworthia or alternatively, that the groups of Haworthia be treated as infrageneric taxa within a broadened (Linnaean) concept of Aloe.
Quasi-periodic nanostructures grown by oblique angle deposition
Authors: Karabacak T. , Wang G.-C. , Lu T.-M.
Published: December 2003

We report that tungsten nanocolumns grown by oblique angle sputter deposition develop a quasi-periodic morphology which is not observed for continuous films deposited at normal incidence. The maximum position in power spectral density of the quasi-periodic nanostructures decreases exponentially as a function of thickness. We explain the formation of the quasi-periodic nature by a “shadowing length” concept which plays a similar role to conventional surface diffusion length. Also, we show that the change of the spatial frequency of the periodicity is a result of the elimination of shorter columns due to the shadowing effect during growth.
Spectroscopic impedance study of nanocrystalline diamond films
Authors: Ye Haitao , Jackman Richard B. , Hing Peter
Published: December 2003

Nanocrystalline diamond films were synthesized by microwave plasma-enhanced chemical vapor deposition using Ar/H2/CH4 gas mixtures. A Fluke PM6306 RCL meter was used to study the electrical impedance of these diamond films in the frequency range 50 Hz to 1 MHz. The impedance dispersion measurement yields the real and imaginary parts in the form of a Cole-Cole plot in the complex plane. A single semicircular response of the impedance of nanocrystalline diamond films was observed at temperatures below 250 °C, with a second semicircular response appearing at low frequencies at temperatures above this. The semicircular responses were found to fit a double resistor-capacitor parallel circuit model. Physical mechanisms likely to be responsible for these observations are discussed in this paper.
Fast relaxation in the structural glass and glassy crystal of ethanol and cyano cyclohexane: A quasielastic light scattering study
Authors: Surovtsev N. V. , Adichtchev S. V. , Wiedersich J. , Novikov V. N. , Rössler E. A.
Published: December 2003

Fast relaxation processes in the structural glass and glassy crystal of ethanol as well as in the glassy crystal of cyano cyclohexane were studied by quasielastic light scattering (LS). Depolarized and polarized LS spectra together with the Brillouin line were measured applying a tandem Fabry–Perot interferometer at frequencies 1–1500 GHz. Depolarized, polarized LS and internal friction data exhibit very similar behavior, thus reflect the same relaxations. The depolarized LS spectra are described by assuming a nearly constant loss (NCL) contribution dominating in the frequency range from 1 GHz to a few tens GHz and by a fast dynamics contribution at higher frequencies. The NCL amplitude increases exponentially with temperature in a similar way as found by dielectric spectroscopy for other molecular glasses. The data cannot be described by thermally activated dynamics in double well potentials, a result contradicting the findings in inorganic glasses such as silica.
Influence of ions on the hydrogen-bond structure in liquid water
Authors: Omta Anne Willem , Kropman Michel F. , Woutersen Sander , Bakker Huib J.
Published: December 2003

The orientational-correlation time of water molecules in ionic solutions has been measured with femtosecond pump–probe spectroscopy. It is found that the addition of ions has no influence on the rotational dynamics of water molecules outside the first solvation shells of the ions. This shows that the presence of ions does not lead to an enhancement or a breakdown of the hydrogen-bond network in liquid water.
A fast-Fourier transform method to solve continuum-electrostatics problems with truncated electrostatic interactions: Algorithm and application to ionic solvation and ion–ion interaction
Authors: Peter Christine , van Gunsteren Wilfred F. , Hünenberger Philippe H.
Published: December 2003

An iterative algorithm based on fast-Fourier transforms is presented that solves the equations of continuum electrostatics for systems of heterogeneous dielectric permittivity (e.g., solute cavity in a solvent) under periodic boundary conditions. The method makes explicit use of the charge–dipole and dipole–dipole interaction tensors, and is thus applicable both to Coulombic interactions (Ewald scheme) and cutoff-based electrostatic interactions described by any polynomial function (including a Coulombic r−1 term), as commonly used in molecular dynamics simulations. The latter case includes, in particular, straight truncation of Coulombic interactions and truncation including a reaction-field correction. After testing and validation by comparison with existing methods, the algorithm is used to investigate the effect of cutoff truncation and artificial periodicity in explicit-solvent simulations of ionic solvation and ion–ion interactions. Both cutoff truncation and artificial periodicity are found to significantly affect the polarization around a spherical ion and its solvation free energy. The nature and magnitude of the two perturbations are analyzed in detail, and approximate analytical correction terms are derived to be applied to the results of explicit-solvent simulations. Cutoff truncation induces strong alterations in the potential of mean force for the interaction between two spherical ions. The present observations based on continuum electrostatics help to rationalize artifacts previously reported from explicit-solvent simulations involving cutoff truncation and, in particular, the unphysical attraction of like charges and repulsion of opposite charges, and the corresponding alterations in the relative stabilities of contact, solvent-separated, and free ion pairs.
Diffusion of 18 elements implanted into thermally grown SiO2
Authors: Francois-Saint-Cyr H. G. , Stevie F. A. , McKinley J. M. , Elshot K. , Chow L. , Richardson K. A.
Published: December 2003

Diffusion data are presented for 18 elements implanted in SiO2 layers thermally grown on silicon and annealed at temperatures ranging from 300 to 1000 °C. Most species studied, (e.g., Be, B, Al, Sc, Ti, V, Zn, Ga, and Mo), showed negligible diffusion over the examined temperature range. In general, this study has shown that the diffusivity of dopants or impurities in SiO2 is significantly smaller than that in silicon. However we also observed that several elements (e.g., Rb and In) have a higher diffusivity in SiO2 than in Si. Because Ga and In are both used as sources for focused ion beam analyses, the lack of Ga diffusion and the movement of In in SiO2 is of interest.
Density and binding forces: Rotational barrier of ethane
Authors: Fernández Rico J. , López R. , Ema I. , Ramı́rez G.
Published: December 2003

The possibility of extending the relationships between density, binding forces, and bonding energies to fine chemical effects is tested taking as an example the rotational barrier of ethane. Electron densities that reasonably fulfill the electrostatic theorem were obtained for several conformations using a Slater basis set. The analysis of these densities shows that the barrier is due to the internal forces acting on the H nuclei. Out of the staggered and eclipsed conformations, the clouds of the hydrogen atoms have a nonskeletal distortion that pull their nuclei toward the staggered conformation.
Heat capacity estimators for random series path-integral methods by finite-difference schemes
Authors: Predescu Cristian , Sabo Dubravko , Doll J. D. , Freeman David L.
Published: December 2003

Previous heat capacity estimators used in path integral simulations either have large variances that grow to infinity with the number of path variables or require the evaluation of first- and second-order derivatives of the potential. In the present paper, we show that the evaluation of the total energy by the T-method estimator and of the heat capacity by the TT-method estimator can be implemented by a finite difference scheme in a stable fashion. As such, the variances of the resulting estimators are finite and the evaluation of the estimators requires the potential function only. By comparison with the task of computing the partition function, the evaluation of the estimators requires k+1 times more calls to the potential, where k is the order of the difference scheme employed. Quantum Monte Carlo simulations for the Ne13 cluster demonstrate that a second order central-difference scheme should suffice for most applications.
Examination of liquid metal surfaces through angular and energy measurements of inert gas collisions with liquid Ga, In, and Bi
Authors: Manning Michelle , Morgan Jason A. , Castro David J. , Nathanson Gilbert M.
Published: December 2003

Gas–liquid scattering experiments are used to measure the recoil directions and energies of neon, argon, and xenon atoms scattering from liquid gallium, indium, and bismuth. The angular and energy distributions vary systematically with the identity of the gas and liquid and with the incident gas energy and liquid temperature. We find that the gas atoms scatter into a narrower angular range from liquids with higher surface tension γ (γGa>γIn>γBi), while they transfer less energy to liquids of higher atomic mass (mBi>mIn>mGa). Comparisons of the angular distributions with scattering models suggest that lower surface tension liquids possess atomically rougher surfaces that redirect impinging atoms more broadly into space. The trend we observe linking broader angular distributions with lower energy transfer appears to be a microscopic manifestation of a general trend between decreasing surface tension and increasing atomic mass for main group metals such as Ga, In, and Bi.
Electron rescattering and the dissociative ionization of alcohols in intense laser light
Authors: Rajgara F. A. , Krishnamurthy M. , Mathur D.
Published: December 2003

The fragmentation dynamics of a series of alcohol molecules, from methanol and ethanol, through hexanol to dodecanol, has been studied by irradiating these molecules with 100 fs duration pulses of linearly and circularly polarized, infrared, intensity-selected laser light. At laser intensities of 1016 W cm−2, the yields of singly and multiply charged atomic fragments from all these molecules are suppressed when circularly polarized light is used. This dependence of the fragmentation dynamics on polarization is rationalized using a simple electron rescattering model. Circular polarization switches “off” electron rescattering and leads to suppression of multiple ionization and molecular fragmentation. The degree of suppression depends upon the amount of energy transfer from the optical field to the molecule: the larger the energy transfer that is required for a particular fragmentation channel, the more marked is its suppression when circular polarization is used. The maximum kinetic energy that is released upon fragmentation appears to be more or less independent of the polarization state of the incident light. The observation that the actual values of kinetic energy released are less than Coulombic indicates that the enhanced ionization mechanism also holds for circularly polarized light.
Formation of neutral and charged gold carbonyls on highly facetted gold nanostructures
Authors: Chau Thoi-Dai , Visart de Bocarmé Thierry , Kruse Norbert , Wang Richard L. C. , Kreuzer Hans Jürgen
Published: December 2003

We show that gold mono- and di-carbonyls are formed on gold field emitter tips during interaction with carbon monoxide gas at room temperature and in the presence of high electrostatic fields. The experiments are done in a time-of-flight atom probe to obtain mass spectra. The yield of monocarbonyl cations is about twice that of di-carbonyl ions. Density functional theory calculations are reported that explain the field stabilization of adsorbed carbonyls and the desorption yield of their cations.
Hydrogen bonding between adsorbed deprotonated glycine molecules on Cu(110)
Authors: Nyberg M. , Odelius M. , Nilsson A. , Pettersson L. G. M.
Published: December 2003

We present new theoretical results indicating the formation of intermolecular hydrogen bonds (H-bonds) between deprotonated glycine molecules (glycinate) on a Cu(110) surface. It is shown that these hydrogen bonds can be probed through the use of x-ray absorption spectroscopy, with surprisingly large effects in the electronic structure. We propose that the (3×2)pg LEED pattern observed for the most stable species of glycinate on the surface is explained by the formation of more than one hydrogen bond per molecule.
Generation–recombination noise in doped-channel Al0.3Ga0.7As/GaAs/In0.2Ga0.8As quantum well micro-Hall devices
Authors: Kunets Vas. P. , Müller U. , Dobbert J. , Pomraenke R. , Tarasov G. G. , Masselink W. T. , Kostial H. , Kissel H. , Mazur Yu. I.
Published: December 2003

The generation–recombination noise in doped-channel quantum-well AlGaAs/GaAs/InGaAs micro-Hall devices is characterized using deep level noise spectroscopy. The source of this low-frequency noise contribution is identified as a single deep level with activation energy of 476 meV. This level is associated with DX centers located in the Al0.30Ga0.70As near the heterointerfaces. A detailed analysis of the experimental data further indicates a trap ionization energy of about 250 meV, an electron capture cross section of about σ0≃1×10−11 cm2, and a total integrated defect concentration of about Nts≃1.4×1010 cm−2.
Structure and dynamics of liquid water adsorbed on the external walls of carbon nanotubes
Authors: Martı́ J. , Gordillo M. C.
Published: December 2003

Molecular dynamics simulations of liquid water near the external surface of a carbon nanotube bundle are presented. Flexible simple point charge and Lennard-Jones potentials have been chosen to model water–water and water–carbon forces, respectively. It has been recently shown that such a system suffers a first-order phase transition between low- and high-density phases. Hydrogen bonding, diffusive behavior, rotational and vibrational motions are analyzed in the low- and high-density regimes.
A comparative study of electron and positron scattering from chlorobenzene (C6H5Cl) and chloropentafluorobenzene (C6F5Cl) molecules
Authors: Makochekanwa C. , Sueoka O. , Kimura M.
Published: December 2003

Total cross sections (TCSs) for electron and positron scattering from chlorobezene (C6H5Cl) and chloropentafluorobenzene (C6F5Cl) molecules have been measured by using the linear transmission time-of-flight method. The impact energies are from 0.4 eV to 1000 eV and 0.2 to 1000 eV for positron and electron impacts, respectively, for scattering from C6H5Cl molecules, and 0.8 eV to 600 eV for electron and 0.4 to 600 eV for positron scattering from C6F5Cl molecules. Two resonance structures are observed in the C6H5Cl electron TCSs at 0.8 eV and 2.5 eV, which merge to the single resonance at 1.6 eV of the C6H6 molecule. This is an evidence of the splitting of the degenerate e2u orbital of the C6H6 molecule into two upon substitution of an H atom by the Cl atom. C6F5Cl electron TCSs are lower than the positron TCSs at 2–4 eV, otherwise elsewhere, electron TCSs for both C6H5Cl and C6F5Cl are greater than positron TCSs by a factor of 1–2 at energies up to 30 eV. In the comparative studies of TCSs for C6H5Cl with C6H6 and C6F5Cl and C6F6, the magnitude of TCSs is found to be in the order of the molecular size over all energy ranges, except positron TCSs for C6H5Cl and C6H6 below 600 eV.
Rotational effects in dissociation of H2 on Pd(111): Quantum and classical study
Authors: Busnengo H. F. , Pijper E. , Kroes G. J. , Salin A.
Published: December 2003

We study rotational effects in dissociation of H2 on Pd(111) through six-dimensional quantum dynamical and classical trajectory calculations. The potential energy surface was obtained from density functional theory. Quantum dissociative adsorption and rotational excitation probabilities are compared with initial-rotational-state-selective measurements. At low energies, dynamic trapping plays an important role, promoting reaction. For low values of the rotational quantum number J, the trapping is mainly due to translation to rotation energy transfer. The decreasing role of trapping when J increases contributes to the decrease of the dissociation probability. For larger values of J trapping is the result of energy transfer to parallel translational motion. Because trapping due to energy transfer to parallel translational motion is only effective at very low energies, the change in trapping mechanism with J causes the minimum of the reaction probability versus collision energy curve to shift to lower energies with increasing J, as previously observed in experiments. Together with dynamic trapping, rotational hindering (for small values of J) and an adiabatic energy transfer from rotation to translation (for high values of J) produce the nonmonotonous dependence of Pdiss on J that is observed in our calculations and experiments at low energies. Finally, we predict a nonmonotonous dependence of the quadrupole alignment A0(2) on J as observed in associative desorption experiments on H2/Pd(100). It is due to rotational hindering for small J and adiabatic energy transfer from rotation to translation for large J.
Finite temperature behavior of impurity doped Lithium cluster, Li6Sn
Authors: Joshi Kavita , Kanhere D. G.
Published: December 2003

We have carried out extensive isokinetic ab initio molecular-dynamic simulations to investigate the finite temperature properties of the impurity doped cluster Li6Sn and the host cluster Li7. The data obtained from about 20 temperatures and total simulation time of at least 3 ns is used to extract thermodynamical quantities like canonical specific heat. We observe that, first, Li6Sn becomes liquidlike around 250 K, at much lower temperature than that for Li7 (≈425 K). Second, a weak shoulder around 50 K in the specific heat curve of Li6Sn is observed due to the weakening of Li–Li bonds. The peak in the specific heat of Li7 is very broad and the specific heat curve does not show any premelting features.
Similarity transformed semiclassical dynamics
Authors: Van Voorhis Troy , Heller Eric J.
Published: December 2003

In this article, we employ a recently discovered criterion for selecting important contributions to the semiclassical coherent state propagator [T. Van Voorhis and E. J. Heller, Phys. Rev. A 66, 050501 (2002)] to study the dynamics of many dimensional problems. We show that the dynamics are governed by a similarity transformed version of the standard classical Hamiltonian. In this light, our selection criterion amounts to using trajectories generated with the untransformed Hamiltonian as approximate initial conditions for the transformed boundary value problem. We apply the new selection scheme to some multidimensional Henon–Heiles problems and compare our results to those obtained with the more sophisticated Herman–Kluk approach. We find that the present technique gives near-quantitative agreement with the the standard results, but that the amount of computational effort is less than Herman–Kluk requires even when sophisticated integral smoothing techniques are employed in the latter.
Practical evaluation of condensed phase quantum correlation functions: A Feynman–Kleinert variational linearized path integral method
Authors: Poulsen Jens Aage , Nyman Gunnar , Rossky Peter J.
Published: December 2003

We report a new method for calculating the Wigner transform of the Boltzmann operator in the canonical ensemble. The transform is accomplished by writing the Boltzmann operator in a semiharmonic form, utilizing the variational centroid effective frequencies introduced by Feynman and Kleinert (FK). The approximate many-body Wigner transformed Boltzmann operator is then utilized with a linearized path integral (LPI) representation for correlation functions. It is shown that this new FK-LPI method is capable of calculating quite accurately the short time behavior of linear and highly nonlinear correlation functions for low temperature Lennard-Jones model systems and that it is vastly superior to classical dynamics. The feasibility of the FK-LPI method for large systems is illustrated by considering a model liquid composed of 32 oxygen molecules with periodic boundary conditions. Initial conditions for molecular dynamics are obtained from its Boltzmann Wigner transform and the FK-LPI method is shown to describe correctly the zero-point motion of the liquid. The effective frequency representation of the Wigner transformed thermal density operator provides an efficient way of sampling nonclassical initial conditions for molecular-dynamics simulations more generally. Applications to vibrational energy relaxation and rate constant calculations in complex molecular systems are discussed.
Recovery of the Smoluchowski–Collins–Kimball kinetics parameters from fluorescence quenching decays
Authors: Kłos Jacek , Molski Andrzej
Published: December 2003

Time-correlated single-photon counting (TCSPC) is a time-resolved fluorescence technique capable of monitoring transient diffusion-mediated kinetics. When the transients can be reliably quantified, TCSPC data can be used for extracting the underlying microscopic dynamics. In the present paper, we study the recovery of the Smoluchowski–Collins–Kimball model parameters from simulated fluorescence quenching decays. A Levenberg–Marquardt least-squares optimization routine was used for the estimation of the sum of the diffusion coefficients of the fluorophore and quencher, D=DF*+DQ, the sum of their radii, R=RF*+RQ, and the intrinsic quenching rate coefficient k. The accuracy and precision of parameter estimation were parameterized by the dimensionless quantities k/4πRD, τD/R2, and 4πR3[Q]/3, where τ is the fluorophore lifetime, and [Q] is the quencher concentration. The zero-time shift was an adjustable parameter. The best parameter estimates are obtained for long-lived fluorophores at high quencher concentrations. The estimated R and D are more accurate as the intrinsic quenching rate k becomes faster, but the estimation of k is optimal when k and the diffusion controlled rate 4πRD are comparable in value. The present study should be useful in planning and interpreting TCSPC experiments on nanosecond and picosecond time scales.
A nonequilibrium Monte Carlo approach to potential refinement in inverse problems
Authors: Wilding Nigel B.
Published: December 2003

The inverse problem for a disordered system involves determining the interparticle interaction parameters consistent with a given set of experimental data. Recently, Rutledge has shown [Phys. Rev. E 63, 021111 (2001)] that such problems can be generally expressed in terms of a grand canonical ensemble of polydisperse particles. Within this framework, one identifies a polydisperse attribute (“pseudospecies”) σ corresponding to some appropriate generalized coordinate of the system to hand. Associated with this attribute is a composition distribution ρ̄(σ) measuring the number of particles of each species. Its form is controlled by a conjugate chemical potential distribution μ(σ) which plays the role of the requisite interparticle interaction potential. Simulation approaches to the inverse problem involve determining the form of μ(σ) for which ρ̄(σ) matches the available experimental data. The difficulty in doing so is that μ(σ) is (in general) an unknown functional of ρ̄(σ) and must therefore be found by iteration. At high particle densities and for high degrees of polydispersity, strong cross coupling between μ(σ) and ρ̄(σ) renders this process computationally problematic and laborious. Here we describe an efficient and robust nonequilibrium simulation scheme for finding the equilibrium form of μ[ρ̄(σ)]. The utility of the method is demonstrated by calculating the chemical potential distribution conjugate to a specific log-normal distribution of particle sizes in a polydisperse fluid.
Anomalous diffusion of vibrational energy in proteins
Authors: Yu Xin , Leitner David M.
Published: December 2003

The evolution of vibrational wave packets built from the normal modes of cytochrome c, myoglobin and green fluorescent protein is investigated. Vibrational energy flow in these proteins is found to exhibit anomalous subdiffusion, a consequence of trapping of energy by spatially localized normal modes contained in the wave packet. Anomalous subdiffusion is characterized by an exponent, ν, that is related to the spectral dimension, d̄, and fractal dimension, D, of the protein. The dispersion relation describing variation of the protein’s normal mode frequencies with wave number is also characterized by an exponent, a, that is related to d̄ and D. Values of the exponent, a, computed for the three proteins are consistent with the computed values for ν. The values of D obtained from ν, a, and d̄ for each protein are the same within computational error, and close to the mass fractal dimension computed for each protein, all values falling in the range D=2.3±0.2. We find also that relaxation of the center of energy of a wave packet computed in terms of normal modes follows stretched exponential kinetics with an exponent equal to 2ν.
A cunning strategy in design of polymeric nanomaterials with novel microstructures
Authors: Huang Lei , He Xuehao , He Tianbai , Liang Haojun
Published: December 2003

A relative approach, based on the dynamic density functional theory, for simulating the solvent evaporation rate dependence of self-assembly process of block copolymers in solution is proposed. The di- and triblock copolymers are first chosen as the candidates for exploration of novel microstructures. The results reveal that asymmetrical block copolymers with unequal block length, which generally exhibit disordered microdomain patterns in melts, have the ability to assemble into periodic ordered microdomain patterns by properly controlling solvent evaporation rate, e.g., diblock copolymers may assemble into lamellar microstructures with lamellar thickness proportional to individual block length. This simulation suggests a strategy of design and manufacture of polymeric nanomaterials with novel microstructures.
Structural quantum isotope effects in amorphous beryllium hydride
Authors: Sampath Sujatha , Lantzky Kristina M. , Benmore Chris J. , Neuefeind Jörg , Siewenie Joan E. , Egelstaff Peter A. , Yarger Jeffery L.
Published: December 2003

The structure factors for amorphous BeD2 and BeH2 were measured using synchrotron x-ray and neutron diffraction techniques. The results show that the structure of amorphous BeD2 is comprised of corner-sharing tetrahedra and is therefore analogous to amorphous H2O and BeF2. A substantial increase in the height of the first sharp x-ray diffraction peak of BeD2 compared to BeH2 is interpreted as a marked increase in the extent of intermediate range order in BeD2 due to stronger network formation. A real-space comparison with liquid water, reveals that the structural isotopic quantum effects are quite different in the two hydrides.
Charge conduction process and photovoltaic properties of a N,N′-di-benzyl 4,4′ bipyridyl dichloride based Schottky device
Authors: Roy M. S. , Manmeeta  , Jaiswal Pratibha , Sharma G. D.
Published: December 2003

Charge conduction process and photovoltaic characteristics of N,N′-di-benzyl 4,4′ bipyridyl dichloride (DBD) based thin film Schottky device were investigated. The conduction mechanism is explained on the basis of modified Schockley equation and through the J–V characteristics under bias voltage ranging from 0.8 to 1.2 V. The space charge limited current (SCLC) region seems to be predominant beyond 1.2 V whereas below 0.8 V the Schottky emission or Poole–Frenkel emission appears to be effective in transportation of current. Impedance analysis was also carried out to ascertain the existence of the Schottky barrier at In–DBD interface. The temperature dependent ac conductivity reveals the predominance of hopping conduction in DBD thin films. The observed higher ideality factor, i.e., 1.65 can be attributed to the high interface resistance leading to low electron mobility and accelerated recombination of electron and hole at In–DBD junction. The photogeneration mechanism of the charge carriers in the device has also been described in detail. The analysis of SCLC at high voltage region and exponential relation of the photocurrent with illumination intensity attributes an exponential trap distribution in the band gap of the film.
Aging process of electrical contacts in granular matter
Authors: Dorbolo S. , Ausloos M. , Vandewalle N. , Houssa M.
Published: December 2003

The electrical resistance decay of a metallic granular packing has been measured as a function of time. This measurement gives information about the size of the conducting cluster formed by the well connected grains. Several regimes have been encountered. Chronologically, the first one concerns the growth of the conducting cluster and is identified to belong to diffusion processes through a stretched exponential behavior. The relaxation time is found to be simply related to the initial injected power. This regime is followed by a reorganization process due to thermal dilatation. For the long-term behavior of the decay, an aging process occurs and enhances the electrical contacts between grains through microsoldering.