bluuebunny/crossref_metadata_2025 huggingface dataset:
Downscaling daily extreme temperatures with genetic programming
Authors: Coulibaly Paulin
Published: August 2004

A context‐free genetic programming (GP) method is presented that simulated local scale daily extreme (maximum and minimum) temperatures based on large scale atmospheric variables. The method evolves simple and optimal models for downscaling daily temperature at a station. The advantage of the context‐free GP method is that both the variables and constants of the candidate models are optimized and consequently the selection of the optimal model. The method is applied to the Chute‐du‐Diable weather station in Northeastern Canada along with the National Center for Environmental Prediction (NCEP) reanalysis datasets. The performance of the GP based downscaling models is compared to benchmarks from a commonly used statistical downscaling model. The experiment results show that the models evolved by the GP are simpler and more efficient for downscaling daily extreme temperature than the common statistical method. The different model test results indicate that the GP approach significantly outperforms the statistical method for the downscaling of daily minimum temperature, while for the maximum temperature the two methods are almost equivalent. However, the GP method remains slightly more effective for maximum temperature downscaling than the statistical method.
A non‐linear statistical downscaling model: El Niño/Southern Oscillation impact on precipitation over New Caledonia
Authors: Fischer Martin , Dewitte Boris , Maîtrepierre Luc
Published: August 2004

A new multivariate non‐linear non‐parametric regression method, called MARS, is used in a statistical downscaling problem for simulating New Caledonian rainfall anomalies induced by El Niño/Southern Oscillation (ENSO). The use of a non‐linear method was motivated by wavelet analyses, pointing to a complex interaction between interannual and intraseasonal rainfall variability. The proposed method is very flexible and computationally efficient. It can be applied to a wide range of problems and its use is not limited to spatial downscaling applications. Furthermore the method provides great potential for comprehensive diagnostics of multivariate problems in climate research. It is shown that this method can take into account the asymmetry between El Niño and La Niña periods and also the type of El Niño.
Modeled response of the Australian monsoon to changes in land surface temperatures
Authors: Wardle Richard , Smith Ian
Published: August 2004

Observations indicate that over the past several decades much of Australia has been getting wetter during summer, while the continent as a whole has been warming. This is consistent with a strengthening of the Australian monsoon. Here we model the climatic response to a reduction in land albedos with the aim of investigating the impacts on the monsoon of changes in land surface temperatures. Changes in the modeled surface energy budget strengthen the monsoon and increase rainfall over the entire continent. Modeled surface temperatures increase in the southern, climatologically drier, regions but decrease in the north. The degree of similarity with the observed rainfall and temperature trends suggests that the strengthening of the monsoon, is independent of changes over the ocean but may be due to changes over the land, highlighting potential sensitivity to changes in climate.
Hydrated states of MgSO4 at equatorial latitudes on Mars
Authors: Feldman W. C. , Mellon M. T. , Maurice S. , Prettyman T. H. , Carey J. W. , Vaniman D. T. , Bish D. L. , Fialips C. I. , Chipera S. J. , Kargel J. S. , Elphic R. C. , Funsten H. O. , Lawrence D. J. , Tokar R. L.
Published: August 2004

The stability of water ice, epsomite, and hexahydrite to loss of H2O molecules to the atmosphere at equatorial latitudes of Mars was studied to determine their potential contributions to the measured abundance of water‐equivalent hydrogen (WEH). Calculation of the relative humidity based on estimates of yearly averages of water‐vapor pressures and temperatures at the Martian surface was used for this purpose. Water ice was found to be sufficiently unstable everywhere within 45° of the equator that if the observed WEH is due to water ice, it requires a low‐permeability cover layer near the surface to isolate the water ice below from the atmosphere above. In contrast, epsomite or hexahydrite may be stable in many near‐equatorial locations where significant amounts of WEH are observed.
Ice core evidence for the extent of past atmospheric CO2 change due to iron fertilisation
Authors: Röthlisberger R. , Bigler M. , Wolff E. W. , Joos F. , Monnin E. , Hutterli M. A.
Published: August 2004

An extended high‐resolution ice core record of dust deposition over the past 60 ka from Dome C, Antarctica, is presented. The data are in conflict with the idea that changes in aeolian iron input into the Southern Ocean were the major cause for the 80 ppm glacial‐interglacial CO2 increase. During the deglaciation, the CO2 increase shows a linear relationship with the fall of the logarithm of the nss‐Ca2+ flux, a proxy for dust deposition. However, the very large variations in the nss‐Ca2+ flux related to the glacial Antarctic warm events A1 to A4 were accompanied by small CO2 variations only. Our data‐based analysis suggests that decreased Southern Ocean dust deposition caused at most a 20 ppm increase in CO2 at the last glacial‐interglacial transition. Rapid decreases in dust deposition to the northern Pacific could have been responsible for a maximum of 8 ppm in addition.
First polar mesosphere summer echoes observed at Davis, Antarctica (68.6°S)
Authors: Morris R. J. , Murphy D. J. , Reid I. M. , Holdsworth D. A. , Vincent R. A.
Published: August 2004

We report the first observations of polar mesosphere summer echoes (PMSE) above the high‐latitude Southern Hemisphere (SH) station Davis, Antarctica (68.6°S, 78.0°E geographic; 74.6°S magnetic). Observations were obtained using a 55 MHz atmospheric radar, the first stage of which was commissioned late in the austral summer of 2002–2003. The radar commenced mesosphere observations with approximately 20 kW of transmitted power in October 2003. PMSE were recorded from 19 November to 3 December 2003 and, after a break in radar operation, from 27 January to 21 February 2004. We present the initial seasonal and diurnal occurrence morphology from 180 hours of Davis PMSE observations. Our initial findings reveal that SH PMSE show similar backscatter echo characteristics and occurrence properties to those reported for the Northern Hemisphere (NH).
Recent changes in the air‐sea gas exchange of methyl chloroform
Authors: Wennberg Paul O. , Peacock Synte , Randerson James T. , Bleck Rainer
Published: August 2004

Atmospheric measurements of methyl chloroform provide important constraints on the rate of oxidation of hydrocarbons in Earth's atmosphere. Estimates of the loss of methyl chloroform to the oceans play a small but important role in these calculations. Here, we examine the ocean‐atmosphere interaction of methyl chloroform in a global ocean model. Contrary to previous assumptions, these simulations suggest that the high‐latitude oceans are currently a source of this chemical to the atmosphere. If confirmed, this finding alters estimates of the change in the atmospheric oxidation rate of hydrocarbons. We highlight the potential usefulness of methyl chloroform as a tracer of ocean circulation.
Titan's atomic and molecular nitrogen tori
Authors: Smith H. T. , Johnson R. E. , Shematovich V. I.
Published: August 2004

Shematovich et al. recently showed plasma induced sputtering in Titan's atmosphere is a source of neutral nitrogen in Saturn's magnetosphere comparable to the photo‐dissociation source. These sources form a toroidal nitrogen cloud roughly centered at Titan's orbital radius but gravitationally bound to Saturn. Once ionized, these particles contribute to Saturn's plasma. When Titan is inside Saturn's magnetopause, newly formed ions can diffuse inward becoming inner magnetospheric energetic nitrogen where they can sputter and be implanted into icy satellite surfaces. Our 3‐D simulation produces the first consistent Titan generated N and N2 neutral clouds; solar UV radiation and magnetospheric plasma subject these particles to dissociation and ionization. The cloud morphologies and associated nitrogen plasma source rates are predicted in anticipation of Cassini data. Since the amount of molecular nitrogen ejected from Titan by photo‐dissociation is small, molecular nitrogen ions detection by Cassini will be an indicator of atmospheric sputtering.
Tidal modulation of Sr/Ca ratios in a Pacific reef coral
Authors: Cohen Anne L. , Sohn Robert A.
Published: August 2004

The strontium‐to‐calcium ratio (Sr/Ca) of reef coral skeleton is an important tool for reconstructing past sea surface temperatures (SSTs). However, the accuracy of paleoSSTs derived from fossil coral Sr/Ca is challenged by evidence that physiological processes influence skeletal chemistry. Here we show that water level variations from tidal forcing are correlated with changes in coral Sr/Ca that cannot be accounted for by changes in SST. Ion microprobe measurements of Sr/Ca ratios in a Pacific Porites lutea reveal high‐frequency variations at periods of ∼6, ∼10, and ∼25 days. The relationship between Sr/Ca and temperature on these short timescales does not follow trends observed at longer periods, indicating that an additional forcing is required to explain our observations. We demonstrate that Sr/Ca is correlated with both tidal water level variations and SST, and that their contributions to the Sr/Ca content of the skeleton vary as a function of period. We propose that water level influences Sr/Ca indirectly via modulation of photosynthetically‐active radiation (PAR) that drives large changes in zooxanthellate photosynthesis.
Hygroscopic and optical properties of organic sea salt aerosol and consequences for climate forcing
Authors: Randles C. A. , Russell L. M. , Ramaswamy V.
Published: August 2004

Scattering of incoming solar radiation by sea salt aerosol is strongly dependent on relative humidity (RH) since hygroscopic particles take up water at high RH. Organic compounds may constitute up to 50% of marine aerosol mass in internal mixtures. We used a detailed thermodynamic and optical model to calculate hygroscopic growth and extinction of sea salt aerosol internally mixed with a soluble organic compound. Increasing organic content from 10 to 50% suppresses growth at high RH compared to a pure NaCl particle by 4 to 20%. For a mildly absorbing organic, the scattering increase with RH is reduced by up to 32% for these mixtures, consistent with observations. Internal mixtures of 90% NaCl and 10% non‐absorbing organics cause 3% less cooling than 100% NaCl particles in the visible spectrum over the clear‐sky oceans. For a mildly absorbing organic compound, 10% organic content reduces radiative cooling substantially compared to 100% NaCl aerosol.
A numerical study of the Southwestern Atlantic Shelf circulation: Barotropic response to tidal and wind forcing
Authors: Palma Elbio D. , Matano Ricardo P. , Piola Alberto R.
Published: August 2004

This article analyzes the barotropic circulation in the Southwestern Atlantic Shelf using a three‐dimensional numerical model forced with winds and tides. South of 40°S, the shelf circulation is dominated by the propagation of the semidiurnal tides. In this region the diurnal tides are generally weak, except at the shelf edge where they resonate with northward propagating, continental shelf waves. North of 40°S, the tidal circulation is relatively weak, and the circulation is mainly driven by the winds. The wind‐driven annual mean circulation is characterized by a broad northeastward flow south of approximately 40°S and is characterized by a southwestward flow farther north. The intense mixing associated with the Patagonian tides enhances the bottom friction that balances the energy input from the wind stress forcing. In contrast with previous results our simulation shows a detrainment of the northward volume transport with latitude due to an offshore flow along the edge of the Patagonian shelf break. The largest seasonal variations of the shelf circulation are observed in the region between 45°S and 25°S where, during the fall, there is a development of a clockwise gyre and a northeastward flow north of 40°S. The gyre weakens toward the winter, and the northeastward flow reverses directions.
Retrieval of stratospheric O3 and NO2 profiles from Odin Optical Spectrograph and Infrared Imager System (OSIRIS) limb‐scattered sunlight measurements
Authors: Haley Craig S. , Brohede Samuel M. , Sioris Christopher E. , Griffioen Erik , Murtagh Donal P. , McDade Ian C. , Eriksson Patrick , Llewellyn Edward J. , Bazureau Ariane , Goutail Florence
Published: August 2004

Scientific studies of the major environmental questions of global warming and ozone depletion require global data sets of atmospheric constituents with relevant temporal and spatial resolution. In this paper, global number density profiles of O3 and NO2 are retrieved from Odin Optical Spectrograph and Infrared Imager System (Odin/OSIRIS) limb‐scattered sunlight measurements, using the maximum a posteriori estimator. Differential optical absorption spectroscopy is applied to OSIRIS radiances as an intermediate step, using the wavelength windows 571–617 nm for O3 and 435–451 nm for NO2. The method is computationally efficient for processing OSIRIS data on an operational basis. Results show that a 2–3 km height resolution is generally achievable between about 12 km and 45 km for O3 with an estimated accuracy of 13% at the peak and between about 15 km and 40 km for NO2 with an estimated accuracy of 10% at the peak. First validations of the retrieved data indicate a good agreement both with other retrieval techniques applied to OSIRIS measurements and with the results of other instruments. Once the validation has reached a confident level, the retrieved data will be used to study important stratospheric processes relevant to global environmental problems. The unique NO2 data set will be of particular interest for studies of nitrogen chemistry in the middle atmosphere.
Rational design of RAR‐selective ligands revealed by RARβ crystal stucture
Authors: Germain Pierre , Kammerer Sabrina , Pérez Efrén , Peluso‐Iltis Carole , Tortolani David , Zusi F Christopher , Starrett John , Lapointe Philippe , Daris Jean‐Paul , Marinier Anne , de Lera Angel R , Rochel Natacha , Gronemeyer Hinrich
Published: August 2004

The crystal structure of the ligand‐binding domain of RARβ, a suspect tumour suppressor, reveals important features that distinguish it from the two other RAR isotypes. The most striking difference is an extra cavity allowing RARβ to bind more bulky agonists. Accordingly, we identified a ligand that shows RARβ selectivity with a 100‐fold higher affinity to RARβ than to α or γ isotypes. The structural differences between the three RAR ligand‐binding pockets revealed a rationale explaining how a single retinoid can be at the same time an RARα, γ antagonist and an RARβ agonist. In addition, we demonstrate how to generate an RARβ antagonist by gradually modifying the bulkiness of a single substitution. Together, our results provide structural guidelines for the synthesis of RARβ‐selective agonists and antagonists, allowing for the first time to address pharmacologically the tumour suppressor role of RARβ in vitro and in animal models.
Dual targeting of plastid division protein FtsZ to chloroplasts and the cytoplasm
Authors: Kiessling Justine , Martin Anja , Gremillon Louis , Rensing Stefan A , Nick Peter , Sarnighausen Eric , Decker Eva L , Reski Ralf
Published: August 2004

FtsZ is a filament‐forming protein that assembles into a ring at the division site of prokaryotic cells. As FtsZ and tubulin share several biochemical and structural similarities, FtsZ is regarded as the ancestor of tubulin. Chloroplasts—the descendants of endosymbiotic bacteria within plant cells—also harbour FtsZ. In contrast to eubacteria, plants have several different FtsZ isoforms. So far, these isoforms have only been implicated with filamentous structures, rings and networks, inside chloroplasts. Here, we demonstrate that a novel FtsZ isoform in the moss Physcomitrella patens is located not only in chloroplasts but also in the cytoplasm, assembling into rings in both cell compartments. These findings comprise the first report on cytosolic localization of a eukaryotic FtsZ isoform, and indicate that this protein might connect cell and organelle division at least in moss.
Between geometry, stability, and polarizability: Density functional theory studies of silicon clusters Sin (n=3–10)
Authors: Pouchan Claude , Bégué Didier , Zhang Daisy Y.
Published: August 2004

The relationship between the polarizability, stability, and the geometry of small-size silicon clusters has been investigated by the density functional theory methods. Results obtained at local density approximation/Vosko-Wilk-Nusair and general gradient approximation/BLYP levels with polarized even-tempered basis set of quadruple ζ quality are presented and compared with those obtained by the B3LYP method, as well as with the ab initio results in recent literature. We have found that the polarizability is directly related to the size of the energy gap between symmetry-compatible bonding and antibonding molecular orbitals, but not necessarily to the size of the HOMO-LUMO (highest occupied molecular orbital–lowest unoccupied molecular orbital) gap. Furthermore, we have defined two structural parameters, namely, the averaged Si-Si distances and the standard deviation of the Si-Si distances, which were found to correlate remarkably well with the binding energy of the clusters and the HOMO-LUMO gap, respectively. These straightforward correlations would, therefore, provide a means to predict the physical properties, in particular, the polarizability and the stability, simply based on the structural information of the cluster.
The orientational order parameters of a dendritic liquid crystal organo-siloxane tetrapode oligomer, determined using polarized infrared spectroscopy
Authors: Merkel K. , Kocot A. , Vij J. K. , Mehl G. H. , Meyer T.
Published: August 2004

The observed macroscopic anisotropic properties such as the components of infrared (IR) absorbances of liquid crystals are expressed in terms of the order parameters of the long molecular axis, molecular, and phase biaxiality. The order parameters of the organo-siloxane tetrapode liquid crystal of zero dendritic order (G0) in its nematic and smectic phases have been determined using results of the polarized IR spectroscopic measurements on a planar homogenously and hometropic aligned cells. The spatial components of the absorbances for the vibrational bands (in the mesogenic unit, terminal chains, and spacer) have been measured and analyzed. For the laboratory reference system, the apparent orientational order parameter S of the mesogen unit shows a significant drop in the transition from the nematic to the smectic phase while the phase biaxiality order parameter P increases to almost 0.4 in the smectic phase. This result shows that the director is tilted out of the sample plane in the smectic phase. The molecular biaxiality parameter D is found to be positive both for the nematic and smectic phases. This suggests that the carbonyl dipoles are oriented close to the tilt plane. For the vibrational bands in the chains, low values of S and D indicative of their low orientational order are obtained. As a result of the interaction among the molecules in the tilted smectic phases, the transition dipoles show positive correlations for the transversal and negative for the longitudinal dipoles.
No additional flow continua in magnetohydrodynamics
Authors: Goedbloed J. P. , Beliën A. J. C. , Holst B. van der , Keppens R.
Published: August 2004

The classical continuous spectrum of fluid flow found by C.M. Case [Phys. Fluids 3, 143 (1960)] is contained in the Alfvén and slow continua of magnetohydrodynamics (MHD). Contrary to several statements in the literature, most recently by S.N. Bhattacharyya and A. Bhattacharjee [Phys. Plasmas 4, 3744 (1997)], there are no additional flow continua in MHD. Additional Eulerian continuum modes do exist in both fluids and plasmas, but they only involve non-Lagrangian perturbations of the density or the entropy, not of the pressure and the magnetic field.
Sheared-flow-driven ion-acoustic drift-wave instability and the formation of quadrupolar vortices in a nonuniform electron–positron–ion magnetoplasma
Authors: Mirza Arshad M. , Azeem M.
Published: August 2004

A system of nonlinear equations which governs the dynamics of low-frequency (in comparison with the ion gyrofrequency) electrostatic waves in a nonuniform electron–positron–ion (e-p-i) magnetoplasma with sheared ion flows is presented. In the linear limit, a dispersion relation is obtained that admits new instabilities of drift-waves. It is found that ion-acoustic and electrostatic drift waves can become unstable due to ion sheared flow. Furthermore, the nonlinear interactions between these finite amplitude short-wavelength waves give rise to quadrupolar vortices. The relevance of the investigation to laboratory and astrophysical plasmas is pointed out.
Modulation of electromagnetic electron cyclotron waves in the presence of nonisothermal electrons in plasmas
Authors: Abbasi H. , Pajouh H. Hakimi , Shukla P. K.
Published: August 2004

Nonlinear interactions between electromagnetic electron cyclotron (EMEC) waves and ion-acoustic perturbations involving nonisothermal electrons and cold inertial ions are considered. Following standard methods, a reduced wave equation for the slowly varying EMEC wave envelope is derived, including nonlinear effects associated with relativistic electron mass variation and electron density changes caused by the EMEC ponderomotive force and trapped electrons in the ion-acoustic perturbations which modulate the EMEC waves. The governing equations are employed to investigate the influence of trapped electrons on the modulational instability and envelope EMEC solitons. It is found that the static magnetic field intensifies the reduction of the growth rate, and makes the conditions for constructing envelope soliton solutions harder.
Selective hyperfine excitation of N2H+ by He: Potential energy surface, cross sections, and propensity rules
Authors: Daniel F. , Dubernet M.-L. , Meuwly Markus
Published: August 2004

We present potential energy surfaces for the He-N2H+ system adiabatically corrected for the zero-point motion along the intermolecular stretching vibrations v1=0 and v1=1. The potentials are extended to shorter He-N2H+ separations which makes them useful for scattering calculations. Close coupling calculations of the spinless S matrices for the rotational excitation of N2H+ by He are presented, and recoupling techniques to obtain collisional excitation cross sections between the N2H+ hyperfine levels are used. The propensity rules between hyperfine levels are investigated for the case where two nuclear spins are involved. It is found that the only well defined propensity rule is ΔF=ΔF1=Δj and that calculations are required in order to obtain the relative intensities of the two-spin hyperfine cross sections.
Control of resistive wall modes in a cylindrical tokamak with radial and poloidal magnetic field sensors
Authors: Finn John M.
Published: August 2004

A simple cylindrical linear magnetohydrodynamic model is introduced to explain recent numerical results relating to stabilization of resistive wall modes by feedback. These older results indicate that feedback with sensors detecting the perturbed poloidal magnetic field is more effective than that with radial field sensors. The model used in this paper allows a complete analytic treatment, so that stability results for both radial and poloidal sensors and the effect of coils that couple poloidal harmonics are transparent. Results with radial sensors and with poloidal sensors, either inside or outside the resistive wall, are compared, showing that the results with internal poloidal sensors are indeed much better than those with radial sensors, if the coupling of poloidal modes by the coils is large. Results with external poloidal sensors are found to be comparable with those with radial sensors. The effect of a phase shift between sensor and control coils is investigated. Sensitivity of the three schemes to high frequency fluctuations, related to sensitivity to noise, is discussed.
Dynamic control and information processing in chemical reaction systems by tuning self-organization behavior
Authors: Lebiedz Dirk , Brandt-Pollmann Ulrich
Published: August 2004

Specific external control of chemical reaction systems and both dynamic control and signal processing as central functions in biochemical reaction systems are important issues of modern nonlinear science. For example nonlinear input–output behavior and its regulation are crucial for the maintainance of the life process that requires extensive communication between cells and their environment. An important question is how the dynamical behavior of biochemical systems is controlled and how they process information transmitted by incoming signals. But also from a general point of view external forcing of complex chemical reaction processes is important in many application areas ranging from chemical engineering to biomedicine. In order to study such control issues numerically, here, we choose a well characterized chemical system, the CO oxidation on Pt(110), which is interesting per se as an externally forced chemical oscillator model. We show numerically that tuning of temporal self-organization by input signals in this simple nonlinear chemical reaction exhibiting oscillatory behavior can in principle be exploited for both specific external control of dynamical system behavior and processing of complex information.
Control of elongation for field-reversed configuration plasmas using axial field index of a mirror confinement field
Authors: Takahashi Tsutomu , Gota Hiroshi , Nogi Yasuyuki
Published: August 2004

The separatrix shape of a field-reversed configuration (FRC) plasma formed by a negative-biased theta-pinch is quite elongated due to a uniform confinement field, except at the end coils (simple mirror field). The elongation of the shape, which is the ratio of half the separatrix length to the separatrix radius, is about 5–10. The control of elongation is attempted by introducing a field gradient along the confinement field (axial field index) over the whole plasma region (smooth mirror field). Since mirror effects act on all portions of the plasma, the FRC plasma is compressed axially and expanded radially. As a result, the elongation becomes almost half that in the simple mirror field. It is found that a poloidal flux in the smooth mirror field is 2.6-fold the value in the simple mirror field.
Turbulence in low-β reconnection
Authors: Ferraro Nathaniel M. , Rogers Barrett N.
Published: August 2004

Using a two-dimensional two-fluid numerical model with finite electron inertia, we have observed turbulence to occur in simulations of plasmas undergoing magnetic reconnection when the plasma β is sufficiently small. This turbulence is caused by secondary instabilities, which are excited by large gradients in the out-of-plane current. These gradients spontaneously form due to the dynamics of ion force balance across the layer, and first arise in the outflow regions downstream from the magnetic x point. This turbulence is not observed to have a substantial effect on the rate of reconnection.
Electron scattering on OH−(H2O)n clusters (n=0–4)
Authors: Svendsen A. , Bluhme H. , Seiersen K. , Andersen L. H.
Published: August 2004

The cross sections for electron scattering on OH−(H2O)n for n=0–4 were measured from threshold to ≈50 eV. All detachment cross sections were found to follow the classical prediction given earlier [Phys. Rev. Lett. 74, 892 (1995)] with a threshold energy for electron-impact detachment that increased upon sequential hydration, yielding values in the range from 4.5 eV±0.2 eV for OH− to 12.10 eV±0.5 eV for OH−(H2O)4. For n⩾1, we found that ≈80% of the total reaction events lead to electron detachment plus total dissociation of the clusters into the constituent molecules of OH and H2O. Finally, we observed resonances in the cross sections for OH−(H2O)3 and for OH−(H2O)4. The resonances were located at ≈15 eV and were ascribed to the formation of dianions in excited states.
Kink instability in electron magnetohydrodynamics
Authors: Jain Neeraj , Das Amita , Kaw Predhiman
Published: August 2004

The paper presents a complete three-dimensional linearized perturbation analysis for a sheared electron flow configuration in the electron magnetohydrodynamic limit. It is shown that the instability criterion in this case is much more relaxed than those of two-dimensional (azimuthally symmetric) modes. It results in a considerable enhancement of the domain of unstable modes in the wave number space. Based on the increased number of unstable modes and the relaxation of the two-dimensional constraint of the existence of two nondissipative square invariants, it can be argued that the three-dimensional case would lead to a highly turbulent state, and may cause significant anomalous damping of the electron flow.
Local properties of quantum chemical systems: The LoProp approach
Authors: Gagliardi Laura , Lindh Roland , Karlström Gunnar
Published: August 2004

A new method is presented, which makes it possible to partition molecular properties like multipole moments and polarizabilities, into atomic and interatomic contributions. The method requires a subdivision of the atomic basis set into occupied and virtual basis functions for each atom in the molecular system. The localization procedure is organized into a series of orthogonalizations of the original basis set, which will have as a final result a localized orthonormal basis set. The new localization procedure is demonstrated to be stable with various basis sets, and to provide physically meaningful localized properties. Transferability of the methyl properties for the alkane series and of the carbon and hydrogen properties for the benzene, naphtalene, and anthracene series is demonstrated.
Density functional theory study on the bridge structure in dimeric aluminum (III) water complexes
Authors: Miao Qiang , Cao Qing , Bi Shuping
Published: August 2004

Density-functional theory methods were used to investigate the structure of dimeric aluminum (III) water complexes as a function of bridging group. The possibilities of oxygen, water, and hydroxyl bridge ligands and a variety of structural arrangements, such as cis/trans, with respect to the relative position of hydroxyl ligands, were considered. Within the limit of our computational level, we found that electrostatic repulsion between hydroxyls is important in deciding the polyaluminum structure. Although the structures of aluminum-hexaaquo predominate, species with small number of charges or a large number of hydroxyl ligands have a tendency toward a five-coordinate trigonal bipyramidal configuration. Because water is electronically neutral, it cannot provide enough negative charges as a bridge ligand to stabilize two Al(III) molecules. The energy differences among many configurational isomers of hydroxyl Al are so small that they may coexist and convert into each other easily at room temperature.
Polymer adsorption onto random planar surfaces: Interplay of polymer and surface correlations
Authors: Polotsky Alexey , Schmid Friederike , Degenhard Andreas
Published: August 2004

We study the adsorption of homogeneous or heterogeneous polymers onto heterogeneous planar surfaces with exponentially decaying site-site correlations, using a variational reference system approach. As a main result, we derive simple equations for the adsorption-desorption transition line. We show that it is preferable to have a small amount of strongly adsorbing sites or monomers rather than a greater amount of weakly adsorbing ones. The results are discussed with respect to their implications for the physics of molecular recognition.
Single particle jumps in a binary Lennard-Jones system below the glass transition
Authors: Vollmayr-Lee K.
Published: August 2004

We study a binary Lennard-Jones system below the glass transition with molecular dynamics simulations. To investigate the dynamics we focus on events (jumps) where a particle escapes the cage formed by its neighbors. Using single particle trajectories we define a jump by comparing for each particle its fluctuations with its changes in average position. We find two kinds of jumps: “reversible jumps,” where a particle jumps back and forth between two or more average positions, and “irreversible jumps,” where a particle does not return to any of its former average positions, i.e., successfully escapes its cage. For all investigated temperatures both kinds of particles jump and both irreversible and reversible jumps occur. With increasing temperature, relaxation is enhanced by an increasing number of jumps and growing jump lengths in position and potential energy. However, the waiting time between two successive jumps is independent of temperature. This temperature independence might be due to aging, which is present in our system. We therefore also present a comparison of simulation data with three different histories. The ratio of irreversible to reversible jumps is also increasing with increasing temperature, which we interpret as a consequence of the increased likelihood of changes in the cages, i.e., a blocking of the “entrance” back into the previous cage. In accordance with this interpretation, the fluctuations both in position and energy are increasing with increasing temperature. A comparison of the fluctuations of jumping particles and nonjumping particles indicates that jumping particles are more mobile even when not jumping. The jumps in energy normalized by their fluctuations are decreasing with increasing temperature, which is consistent with relaxation being increasingly driven by thermal fluctuations. In accordance with subdiffusive behavior are the distributions of waiting times and jump lengths in position.
Low-energy impact of X−(H2O)n (X=Cl,I) onto solid surface
Authors: Koizumi Shin’ichi , Yasumatsu Hisato , Otani Shigeki , Kondow Tamotsu
Published: August 2004

We investigated dissociation of X−(H2O)n (X=Cl, I, n=13–31) by the impact onto a (La0.7Ce0.3)B6(100) surface at a collision energy Ecol of 1–5 eV per water molecule in a tandem time-of-flight mass spectrometer equipped with a translation-energy analyzer. The mechanism of the dissociation was elucidated on the basis of the measurements of the mass spectrum and the translational energies of the product anions, X−(H2O)m (m=0–4), scattered from the surface. It was concluded that (1) the parent cluster anion impacted on the surface undergoes dissociation on the surface under quasiequilibrium with its characteristic time varying with Ecol and n, and (2) the total collision energy introduced is partitioned preferentially to the translational motions of the products on the surface and to the rotational, the vibrational, and the lattice vibrational motions (surface) in this order. The quasiequilibrium model is applicable, even at the collision energy as low as 1 eV, because the translational modes are found to be statistically distributed while the other modes are not much populated by dynamical and energetics limitation.
Surface ordering of diskotic liquid crystals
Authors: Bellier-Castella L. , Caprion D. , Ryckaert J.-P.
Published: August 2004

We present Monte Carlo simulations of diskotic molecules using the Gay-Berne potential in a slab geometry. The disk-wall interaction is described by two different functions according to whether or not the equilibrium distance is dependent on the relative orientation of the disk to the wall. Furthermore, by changing the parameters of these potentials, we model either homeotropic (face-on) or planar (edge-on) anchoring of the disks. We have found that the isotropic-nematic transition does not change in comparison with the bulk situation. The temperature of the nematic-columnar transition, on the contrary, is found to increase for homeotropic anchoring, and decrease for planar anchoring, independently of the details of the potential. We explain the decrease of the transition temperature in the planar anchoring situation as the result of an induced frustration, due to the competition between the two orientations induced independently by the upper and lower walls.
Electron scattering from tetrafluoroethylene
Authors: Panajotovic R. , Jelisavcic M. , Kajita R. , Tanaka T. , Kitajima M. , Cho H. , Tanaka H. , Buckman S. J.
Published: August 2004

We report experimental results for electron scattering from tetrafluoroethylene, C2F4, obtained from measurements in two laboratories. An extensive set of differential, integral, and momentum transfer cross sections is provided for elastic scattering for incident electron energies from 1 to 100 eV and inelastic (vibrational excitation) scattering for incident electron energies at 3, 6, 7.5, 8, and 15 eV, and for scattering angles ranging from 10° to 130°. To highlight the role of intermediate negative ions (resonances) in the scattering process we have also measured excitation functions for elastic scattering and vibrational excitation of the ground electronic state of C2F4 for incident energies between 1.5 and 20 eV. Our results are compared with recent theoretical calculations and a limited number of other experimental results.
Dispersion relation for planar rippled wall backward wave oscillator driven by sheet electron beam
Authors: Gokhale A. , Mondal J. , Mittal K. C. , Choyal Y. , Maheshwari K. P.
Published: August 2004

A dispersion relation is derived for an intense sheet electron beam driven by planar rippled wall slow wave structure. The dispersion characteristics of the interaction, such as the linear instability growth rate and spatial growth rate, are also analyzed numerically.
Two-dimensional dynamics of two-component plasma with finite β
Authors: Tsidulko Yu. A.
Published: August 2004

The collisionless two-dimensional dynamics of a two-component plasma, where the fast ions determine the plasma pressure and the cold ions determine the plasma inertia, is studied both analytically and numerically. The fast ion Larmor radius is assumed to be of the order of the plasma transverse size. The set of equations for such a plasma is obtained, taking into account small but finite plasma pressure (β≪1). It is shown that, in the limit of high cold component density, the dynamics studied in this paper is qualitatively similar to Euler two-dimensional hydrodynamics. The existence of an instability due to nonuniform gradient drift is shown. This instability is similar to the Rayleigh instability in hydrodynamics and the diocotron instability in a nonneutral plasma. Some analytical samples of eigen modes are presented. The stability of axisymmetric equilibrium with a monotone profile of the guiding center density is proved. The ensemble of the large Larmor radius ions with conserved magnetic moments has a potential energy, which reduces when the spatial distribution of the guiding centers becomes more compact. The transformation of this energy to the kinetic energy of E×B turbulence is accompanied by self-localization of the fast ion spatial distribution, which is demonstrated by the numerical simulation.
Interactions between colloidal particles in polymer solutions: A density functional theory study
Authors: Patel N. , Egorov S. A.
Published: August 2004

We present a density functional theory study of colloidal interactions in a concentrated polymer solution. The colloids are modeled as hard spheres and polymers are modeled as freely jointed tangent hard sphere chains. Our theoretical results for the polymer-mediated mean force between two dilute colloids are compared with recent simulation data for this model. Theory is shown to be in good agreement with simulation. We compute the colloid-colloid potential of mean force and the second virial coefficient, and analyze the behavior of these quantities as a function of the polymer solution density, the polymer chain length, and the colloid/polymer bead size ratio.
Structures, energetics, and spectra of hydrated hydroxide anion clusters
Authors: Lee Han Myoung , Tarkeshwar P. , Kim Kwang S.
Published: August 2004

The structures, energetics, electronic properties, and spectra of hydrated hydroxide anions are studied using density functional and high level ab initio calculations. The overall structures and binding energies are similar to the hydrated anion clusters, in particular, to the hydrated fluoride anion clusters except for the tetrahydrated clusters and hexahydrated clusters. In tetrahydrated system, tricoordinated structures and tetracoordinated structures are compatible, while in pentahydrated systems and hexahydrated systems, tetracoordinated structures are stable. The hexahydrated system is similar in structure to the hydrated chloride cluster. The thermodynamic quantities (enthalpies and free energies) of the clusters are in good agreement with the experimental values. The electronic properties induced by hydration are similar to hydrated chloride anions. The charge-transfer-to-solvent energies of these hydrated-hydroxide anions are discussed, and the predicted ir spectra are used to explain the experimental data in terms of the cluster structures. The low-energy barriers between the conformations along potential energy surfaces are reported.
The flow in a cylindrical container with a rotating end wall at small but finite Reynolds number
Authors: Muite Benson K.
Published: August 2004

Similarity solutions of the first kind are intermediate asymptotic solutions for the Stokes flow field and for the first-order inertial correction to the Stokes flow field in small aspect ratio geometries with both no-slip and free-slip boundary conditions opposite the rotating end wall. These results differ from the semi-infinite cylindrical container, where similarity solutions of the second kind are intermediate asymptotic representations of the Stokes and first-order flow fields. Lanczos factors are used to show that for Reynolds numbers less than 1, the boundary discontinuity has a limited influence on the flow field with a no-slip boundary condition opposite the rotating end wall but the boundary discontinuity is important in determining the flow field with a free-slip boundary condition opposite the rotating end wall.
Dissolution of a base (RbOH) by water clusters
Authors: Odde Srinivas , Lee Han Myoung , Kołaski Maciej , Mhin Byuing Jin , Kim Kwang S.
Published: August 2004

Density functional and ab intio calculations are employed in order to understand the base dissociation of rubidium hydroxide by water molecules. The hydrated structures, stabilities, thermodynamic quantities, dissociation energies, infrared spectra, and electronic properties of RbOH(H2O)n=0–5 are investigated. With the successive addition of water molecules to RbOH, the Rb-OH bond lengthens significantly from 2.45 Å for n=0 to 3.06 Å for n=5. It is interesting to note that four water molecules are needed for the stable dissociation of RbOH (as an almost dissociate conformation) and five water molecules are needed for the complete dissociation without any Rb-OH stretch mode, in contrast to the same group base of CsOH which requires only three water molecules for an almost dissociate conformation and four water molecules for the complete dissociation.
Mechanism analysis of controlled quantum dynamics in the coordinate representation
Authors: Sharp Richard W. , Rabitz Herschel
Published: August 2004

There are increasing numbers of successful numerical simulations as well as laboratory tests of controlled quantum phenomena. However, little is understood about the control mechanisms, especially in strongly driven systems. In this paper, mechanism is defined in terms of the significant quantum pathways induced by the control field, connecting the initial state to the final target state. The present work focuses on understanding mechanism in the coordinate representation, which is natural for many dynamical systems. The quantum amplitudes defining the mechanistic pathways are determined by a special procedure called coding Hamiltonians to access mechanistic pathways (CHAMP). Numerical procedures are presented for performing CHAMP calculations in a stable fashion. Mechanism determination is illustrated for excitation of a model linear triatomic molecule. Mechanisms are revealed for optimal control fields as well as for suboptimally driven systems including snapshots in time of the evolving mechanism and the effect of field noise on the mechanism. The high order multiphoton processes involved in the example produce many interfering pathways to create the final state, yet the dynamics and the mechanism are robust to reasonable levels of noise.
Rotational energy transfer in collisions between CO(X 1Σ+, v=2, J=0, 1, 4, and 6) and He at temperatures from 294 to 15 K
Authors: Carty David , Goddard Andrew , Sims Ian R. , Smith Ian W. M.
Published: August 2004

Infrared-vacuum ultraviolet double resonance experiments have been implemented in the ultracold environment provided by a Cinétique de Réaction en Ecoulement Supersonique Uniforme apparatus. With this technique rate coefficients of two kinds have been measured for rotational energy transfer in collisions between CO and He: (a) those for total removal from the selected rotational states J=0, 1, 4, and 6 in the vibronic state X 1Σ+, v=2, and (b) those for transfer between selected initial and specific final states. Using different Laval nozzles, results have been obtained at several different temperatures: 294, 149, 63, 27, and 15 K. The thermally averaged cross sections for total removal by collisions with He show only slight variations both with initial rotational state and with temperature. The variation of state-to-state rate coefficients with ΔJ show several general features: (i) a decrease with increasing ΔJ; (ii) a propensity to favor odd ΔJ over even ΔJ; and (iii) at lower temperatures, the distribution of rate coefficients against ΔJ becomes narrower, and decreases in J are increasingly favored over increases in J, a preference which is most strongly seen for higher initial values of J. The results are shown to be in remarkably good agreement with those obtained in ab initio scattering calculations by Dalgarno and co-workers [Astrophys. J. 571, 1015 (2002)].
Polarization spectroscopy of gaseous tropolone in a strong electric field
Authors: Wu Chengyin , He Yonggang , Kong Wei
Published: August 2004

We report studies of polarization spectroscopy of gaseous tropolone in a strong electric field using resonantly enhanced multiphoton ionization. The electric field induces localization of the tunneling proton between the two equivalent oxygen atoms. As a result, the C2v symmetry of the molecular frame is broken, and the parity selection rule is violated. The field induced transitions are type A with transition dipoles perpendicular to those under field free conditions. The polarization ratios, i.e., the ratios of the overall excitation yield under different polarizations of the resonant laser, thus deviate from those of a pure type B transition. In a field of 60 kV/cm, the experimental polarization ratio implies an essentially equal mixture of type B and type A transitions. Moreover, the induced transitions overlap with the two field-free subbands, and the resulting intensity ratios between the two subbands demonstrate dependence on the applied electric field. These observations can be qualitatively modeled using a quantum mechanical approach by assuming a two level system. A puzzling result is the magnitude of the transition dipole of the induced transition, which is proven to be essentially linearly dependent on the applied electric field.