bluuebunny/crossref_metadata_2025 huggingface dataset:
Specification of the dispersion coefficient in the modeling of gravity‐driven flow in porous media
Authors: Kalejaiye Bolarinwa O. , Cardoso Silvana S. S.
Published: October 2005

Dispersion is an important component of transport in gravity‐driven flow in porous media. However, its detailed modeling requires large computer powers and long calculation times. In this paper, we theoretically and experimentally investigate the conditions for which dispersion may be treated (1) as a simple constant or (2) as a more complex velocity‐dependent term for flow in a homogeneous porous medium. The fundamental equations for fluid flow and solute transport in a porous medium are solved using an augmented Lagrangian algorithm. A general numerical code is developed in which the dispersion coefficient can be set as either a constant or as velocity‐dependent. The theoretical results predicted by these two models are compared with those of our new laboratory experiments. We also theoretically analyze case scenarios not easily realizable in the laboratory. On the basis of these results we propose a method for estimating representative constant dispersion coefficients and discuss the conditions for which such simplification is valid.
Influence of reduced carbon emissions and oxidation on the distribution of atmospheric CO2: Implications for inversion analyses
Authors: Suntharalingam Parvadha , Randerson James T. , Krakauer Nir , Logan Jennifer A. , Jacob Daniel J.
Published: October 2005

Recent inverse analyses constraining carbon fluxes using atmospheric CO2 observations have assumed that the CO2 source from atmospheric oxidation of reduced carbon is released at the surface rather than distributed globally in the atmosphere. This produces a bias in the estimates of surface fluxes. We used a three‐dimensional (3D) atmospheric chemistry model (GEOS‐CHEM) to evaluate the magnitude of this effect on modeled concentrations and flux estimates. We find that resolving the 3D structure of the atmospheric CO2 source, as opposed to emitting this reduced carbon as CO2 at the surface, yields a decrease in the modeled annual mean interhemispheric gradient (N‐S) of 0.21 ppm. Larger adjustments (up to −0.6 ppm) are apparent on a regional basis in and downwind of regions of high reduced carbon emissions. We used TransCom3 annual mean simulations from three transport models to evaluate the implications for inversion estimates. The main impacts are systematic decreases in estimates of northern continental land uptake (i.e., by 0.22 to 0.26 Pg C yr−1), and reductions in tropical land carbon efflux with smaller changes over oceans and in the Southern Hemisphere. These adjustments represent a systematic bias in flux estimates, accounting for changes of 9 to 27% in the estimated northern land CO2 sink for the three models evaluated here. Our results highlight the need for a realistic description of reduced carbon emission and oxidation processes in deriving inversion estimates of CO2 surface fluxes.
Seasonal variations of CO and HCN in the troposphere measured by solar absorption spectroscopy over Poker Flat, Alaska
Authors: Kasai Yasuko J. , Kagawa Akiko , Jones Nicholas , Fujiwara Akimitsu , Seki Koji , Murayama Yasuhiro , Murcray Frank
Published: October 2005

Tropospheric partial column abundances of CO and HCN have been retrieved from infrared solar spectra observed with a ground‐based spectrometer at Poker Flat Alaska (65°N, 147°W) over the time period from 2000 to 2004. From these data we report the transpacific transport induced inter‐annual variability of tropospheric CO over Poker Flat. This is the first report of solar infrared data from the Poker Flat station, where the geographical location of the site means that remote sensing measurements are sampling the transport of transpacific air parcels going to Northern America from Eastern Siberia and Asia. The five year time‐span of the data also show significant differences in year to year CO and HCN tropospheric column enhancements driven by changes in Siberian/Asian pollution sources.
Marine productivity estimates from continuous O2/Ar ratio measurements by membrane inlet mass spectrometry
Authors: Kaiser Jan , Reuer Matthew K. , Barnett Bruce , Bender Michael L.
Published: October 2005

Dissolved oxygen/argon (O2/Ar) ratios in the oceanic mixed layer are indicative of net community production (NCP) because O2 and Ar share similar physical solubility properties, but only O2 is biologically produced and consumed. We describe a membrane inlet mass spectrometer (MIMS) that allows continuous high‐precision shipboard analysis of O2/Ar ratios and eventually other gases, calibrated with discrete samples analyzed in the laboratory. We also present O2/Ar data from the eastern equatorial Pacific. Short‐term reproducibilities of 0.05% were achieved. Meridional gradients and small‐scale phenomena were clearly resolved. O2/Ar undersaturations around the equator reflect the interaction of biological and physical forcings. Mixed‐layer NCP estimated from wind speed‐gas exchange parameterizations was near zero north of 2.75°N, and about 12 mmol m−2 d−1 south of 6.75°S. Ar supersaturations, calculated from MIMS O2/Ar measurements and accompanying O2 concentration measurements, ranged from −0.8 to +3.0%.
On latitudinal profiles of zonal means
Authors: Benestad R. E.
Published: October 2005

A well‐known effect of the near‐spherical shape of Earth is that longitude‐latitude map projections result in a spatial stretching of the information in the zonal direction in high latitudes. It is also well known that the variance of mean value estimates of climatic data varies inversely with the size of the area or length of interval over which the means are taken. A perhaps lesser‐known implication is that the real degrees of freedom in the zonal mean value vary with latitude. Therefore taking zonal means for different latitudes involves averaging over different number of truly independent observations. It is shown here that a comparison between zonal mean values from different latitudes can give misleading results if the effect of latitudinal varying real degrees of freedom is not properly accounted for.
Assessing the impact of changes in climate and CO2 on potential carbon sequestration in agricultural soils
Authors: Jain Atul K. , West Tristram O. , Yang Xiaojuan , Post Wilfred M.
Published: October 2005

Changes in soil management can potentially increase the accumulation of soil organic carbon (SOC), thereby sequestering CO2 from the atmosphere. However, the amount of carbon sequestered in soils can be augmented or lessened due to changes in climate and atmospheric CO2 concentration. The purpose of this paper is to study the influence of climate and CO2 feedbacks on soil carbon sequestration using a terrestrial carbon cycle model. Model simulations consist of observed adoption rates of no‐tillage practices on croplands in the U.S. and Canada between 1981–2000. Model results indicate potential sequestration rates between 0.4–0.6 MgC/ha/yr in the Midwestern U.S. with decreasing rates towards the western, dryer regions of the U.S. It is estimated here that changes in climate and CO2 between 1981–2000 could be responsible for an additional soil carbon sequestration of 42 Tg. This is 5% of the soil carbon estimated to be potentially sequestered as the result of conversion to no‐tillage in the U.S. and Canada.
Quantification of the radar reflectivity sampling error in non‐stationary rain using paired disdrometers
Authors: Berne A. , Uijlenhoet R.
Published: October 2005

Knowledge of the raindrop size distribution (DSD) is essential for understanding the physics of precipitation and for interpreting remotely sensed observations of rain. Disdrometer measurements of DSDs are affected by uncertainties due to the limited sampling volumes or areas of the sensors. Determining this sampling error directly from disdrometer observations is of primary importance for the practical application of DSD analyses. Gage et al. (2004) proposed an estimator of the sampling error affecting the radar reflectivity estimates based on pairs of collocated disdrometers. We provide an interpretation of this estimator and assess its accuracy through controlled experiments using a Monte Carlo framework. Our simulation model of the disdrometer sampling process closely mimics the observations reported by Gage et al. (2004). Using this model, we demonstrate that the estimator proposed by Gage et al. (2004) provides a reliable quantification of the reflectivity sampling error. However, we also show that its accuracy depends on the ratio between the length of the disdrometer time series involved and the characteristic time scale of the rainfall.
Early deglacial (∼19–17 ka) strengthening of the northeast monsoon
Authors: Tiwari Manish , Ramesh R. , Somayajulu B. L. K. , Jull A. J. T. , Burr G. S.
Published: October 2005

Earlier studies that mostly relied on bulk 14C dating indicated a weaker Southwest monsoon (SWM) and a stronger than present Northeast monsoon (NEM) during the Last Glacial Maximum (LGM, ∼18000 14C years or ∼21000 calendar years ago). We have analyzed a sediment core from the equatorial Indian Ocean, where planktonic foraminifera were dated by Accelerator Mass Spectrometry (AMS 14C). δ18O analysis of three species of foraminifera viz. Globigerinoides ruber, Globigerinoides sacculifer, and Globorotalia menardii reveals that the NEM strengthened during the early deglacial period (∼15000 14C years ago or ∼18000 calendar years ago) relative to the present and not during the LGM as surmised earlier.
Ionospheric irregularities in the low‐latitude valley region observed with the Equatorial Atmosphere Radar
Authors: Yokoyama T. , Patra A. K. , Fukao S. , Yamamoto M.
Published: October 2005

The geomagnetically low‐latitude valley region between the upper E region and the lower F1 region is studied with the Equatorial Atmosphere Radar (EAR) in Indonesia. Three‐meter‐scale field‐aligned irregularity echoes have been frequently observed in the valley region in association with the equatorial spread F (ESF) in the period from sunset to midnight. The valley region echoes usually appear at above 150 km and propagate downward with time. Rapid beam scanning of the EAR revealed that spatial structure, temporal variation, and drift velocity of the valley region echoes resemble those of ESF, which indicates that the dynamoelectric field in the equatorial F region controls the low‐latitude valley region irregularities. Perturbed electric fields associated with ESF map down to the low‐latitude valley region and can produce the perturbed plasma density structures as “images” of ESF structures. Image structure is effectively formed at altitudes below 200 km and is a source of the valley region irregularities observed with the EAR. It is suggested that intermediate layers should supply a plasma density gradient for excitation of 3‐m‐scale irregularities in the valley region through gradient drift instability.
Diagnosing the stratosphere‐to‐troposphere flux of ozone in a chemistry transport model
Authors: Hsu Juno , Prather Michael J. , Wild Oliver
Published: October 2005

Events involving stratosphere‐troposphere exchange (STE) of ozone, such as tropopause folds and westerly ducts, are readily identified in observations and models, but a quantitative flux specifying where and when stratospheric ozone is mixed into the troposphere is not readily discerned from either. This work presents a new diagnostic based on determining when stratospheric air is mixed and diluted down to tropospheric abundances (<100 ppb) and hence effectively participates in tropospheric chemistry. The method is applied to two years of high‐resolution, global meteorological fields (1.9 degrees, 40 levels) from the ECMWF forecast model derived by U. Oslo for chemistry transport modeling and used in TRACE‐P studies. The UCI CTM is run here with linearized stratospheric ozone chemistry (Linoz) and a parameterized tropospheric sink. In terms of events, the CTM accurately follows a March 2001 westerly duct stratospheric intrusion into the tropical eastern Pacific as observed by TOMS and calculates a 48‐hour burst of STE O3 flux for that region. The influx associated with the event (0.3 Tg) is much less than the anomalous amount seen as an isolated island in column ozone (1.7 Tg). A climatology of monthly mean STE fluxes is similar for both years (January to December 1997 and May 2000 to April 2001), but the warm phase of ENSO December 1997 is distinctly different from the cold phase of ENSO month December 2000. Global ozone fluxes are about 515 Tg (year 1997) and 550 Tg (year 2000/2001) with an equal amount into each hemisphere, and larger springtime fluxes for both hemispheres. In terms of geographical distribution, Northern Hemisphere regions of high ozone flux follow the jet streams over the oceans in the winter and over the continents in the summer, in agreement with many previous studies. In contrast, we find the largest STE flux is located in the subtropics during late spring, particularly over the Tibetan Plateau in May. This hot spot of STE is not a numerical artifact, it occurs in both meteorological years, and it appears to be caused by the rapid erosion of the tropopause. Ozone fluxes in the Southern Hemisphere have less variability (either temporal or spatial), and they occur mainly in the subtropical region (25°S–35°S) regardless of season. The poles, throughout the year, show minimal STE O3 flux.
Possible formation of ancient crust on Mars through magma ocean processes
Authors: Elkins‐Tanton Linda T. , Hess Paul C. , Parmentier E. M.
Published: October 2005

Models for Martian magma oceans of varying depths predict that decompression mantle melting, perhaps forming Mars' earliest crust, could occur during gravitationally driven solid‐state overturn of cumulates following magma ocean solidification. When hot cumulates rise from depth during solid‐state overturn, some regions melt adiabatically, producing basaltic to andesitic magmas. The resulting crust would be formed at between 30 and 50 Myr after planetary accretion, when magma ocean solidification and subsequent overturn are complete. Models of magma oceans deeper than ∼1550 km consistently produce two separate magmatic source regions during overturn that create compositionally distinct magmas, consistent with both major and trace element data for SNC meteorites and the Martian crust. In a partial magma ocean between ∼1550 and ∼1250 km (∼15 GPa) the only early magma produced is from a shallow pyroxene + olivine source; but if the magma ocean were less than ∼1150 km (∼14 GPa) deep, the underlying (undifferentiated or minimally differentiated) mantle rises sufficiently during overturn that it melts adiabatically and produces an early magma. Magma ocean models therefore produce specific predictions for the volumes and compositions of the most ancient crust produced by a range of initial magma ocean depths. The predicted crustal compositions and volumes for a whole mantle magma ocean are consistent with observations of Mars today.
Structural characteristics of shallowly buried accretionary prism: Rapidly uplifted Neogene accreted sediments on the Miura‐Boso Peninsula, central Japan
Authors: Yamamoto Y. , Mukoyoshi H. , Ogawa Y.
Published: October 2005

The upper Miocene Misaki and Nishizaki formations on the Miura and Boso peninsulas in central Japan preserve the deformation features of an off‐scraped accretionary prism. The spatial distribution, geometry, and style of accretion‐related deformation with paleotemperature and burial depth estimation are elucidated in this study. The deformation structures and textures are similar to those of modern accretionary prisms. The low maximum paleotemperature (<50°C) and high preserved porosity of the sediments (30–50%) imply a maximum burial depth of less than 1000 m. On the basis of the mode of deformation, this off‐scraped body is divided into an imbricate thrust, a thrust unit, and an upper coherent unit in ascending order. The imbricate thrust corresponds to a branch from the basal décollement zone and is subdivided into a brecciated zone, a main gouge zone, and a shear band zone. The thrust unit hosts a concentration of thrust systems that form various orders of duplex structures, while the upper coherent unit is characterized by gravitational instability‐ and earthquake‐induced deformation without thrust faulting. The duplex distribution, shear strain, and fluid migration associated with the off‐scraping processes are clearly localized within the imbricate thrust and thrust unit. This accretion‐related deformation occurred before lithification of the sediment and under high fluid pressure induced by shear deformation, thickening of the sedimentary sequence, and earthquake‐induced liquefaction. These processes are inferred to control the effect on the mode of deformation and the location of thrusting during early deformation of the accretionary prism.
Multiobjective management of ecosystem services by integrative watershed modeling and evolutionary algorithms
Authors: Bekele Elias G. , Nicklow John W.
Published: October 2005

This paper explores the role of landscapes in generating ecosystem services while maximizing gross margin associated with agricultural commodity production. Ecosystem services considered include the reduction of nonpoint source pollutants such as sediment, phosphorous, and nitrogen yields from a watershed. The analysis relies on an integrative modeling framework that combines a comprehensive watershed model (SWAT) with a multiobjective evolutionary algorithm (SPEA2). Application of the resulting model to a watershed in southern Illinois demonstrates the effectiveness of the approach in providing tradeoff solutions between gross margin and the generation of ecosystem services. These solutions are important to policy makers and planners in that they provide information about the cost‐effectiveness of alternative agricultural landscapes.
The State of Psychological Ownership: Integrating and Extending a Century of Research
Authors: Pierce Jon L. , Kostova Tatiana , Dirks Kurt T.
Published: October 2005

People develop feelings of ownership for a variety of objects, material and immaterial in nature. We refer to this state as psychological ownership. Building on and extending previous scholarship, the authors offer a conceptual examination of this construct. After defining psychological ownership, they address “why” it exists and “how” it comes into being. They propose that this state finds its roots in a set of intraindividual motives (efficacy and effectance, self-identity, and having a place to dwell). In addition, they discuss the experiences that give rise to psychological ownership and propose several positive and negative consequences of this state. The authors’ work provides a foundation for the development of a comprehensive theory of psychological ownership and the conceptual underpinnings for empirical testing.
Optimal control of ultrafast cis-trans photoisomerization of retinal in rhodopsin via a conical intersection
Authors: Abe Mayumi , Ohtsuki Yukiyoshi , Fujimura Yuichi , Domcke Wolfgang
Published: October 2005

Optimal control simulation is applied to the cis-trans photoisomerization of retinal in rhodopsin within a two-dimensional, two-electronic-state model with a conical intersection [S. Hahn and G. Stock, J. Phys. Chem. B 104, 1146 (2000)]. For this case study, we investigate coherent control mechanisms, in which laser pulses work cooperatively with a conical intersection that acts as a “wave-packet cannon.” Optimally designed pulses largely consist of shaping subpulses that prepare a wave packet, which is localized along a reaction coordinate and has little energy in the coupling mode, through multiple electronic transitions. This shaping process is shown to be essential for achieving a high target yield although the envelopes of the calculated pulses depend on the local topography of the potential-energy surfaces around the conical intersection and the choice of target. The control mechanisms are analyzed by considering the motion of reduced wave packets in a nuclear configuration space as well as by snapshots of probability current-density maps.
Geometry of C-flat connections, coarse graining and the continuum limit
Authors: Martínez Jorge , Meneses Claudio , Zapata José A.
Published: October 2005

A notion of effective gauge fields which does not involve a background metric is introduced. The role of scale is played by cellular decompositions of the base manifold. Once a cellular decomposition is chosen, the corresponding space of effective gauge fields is the space of flat connections with singularities on its codimension two skeleton, AC-flat∕G¯M,⋆⊂A¯M∕G¯M,⋆. If cellular decomposition C2 is finer than cellular decomposition C1, there is a coarse graining map πC2→C1:AC2-flat∕G¯M,⋆→AC1-flat∕G¯M,⋆. We prove that the triple (AC2-flat∕G¯M,⋆,πC2→C1,AC1-flat∕G¯M,⋆) is a principal fiber bundle with a preferred global section given by the natural inclusion map iC1→C2:AC1-flat∕G¯M,⋆→AC2-flat∕G¯M,⋆. Since the spaces AC-flat∕G¯M,⋆ are partially ordered (by inclusion) and this order is directed in the direction of refinement, we can define a continuum limit, C→M. We prove that, in an appropriate sense, limC→MAC-flat∕G¯M,⋆=A¯M∕G¯M,⋆. We also define a construction of measures in A¯M∕G¯M,⋆ as the continuum limit (not a projective limit) of effective measures.
Real trajectories in the semiclassical coherent state propagator
Authors: Novaes Marcel
Published: October 2005

The semiclassical approximation to the coherent state propagator requires complex classical trajectories in order to satisfy the associated boundary conditions, but finding these trajectories in practice is a difficult task that may compromise the applicability of the approximation. In this work several approximations to the coherent state propagator are derived that make use only of real trajectories, which are easier to handle and have a more direct physical interpretation. It is verified in a particular example that these real trajectories approximations may have excellent accuracy.
Fragmentation dynamics of condensed phase thymine by low-energy (10–200eV) heavy-ion impact
Authors: Deng Zongwu , Imhoff Marjorie , Huels Michael A.
Published: October 2005

We report measurements of the formation and desorption of ionic fragments induced by very low-energy (10–200eV) Ar+ irradiation of thymine (T) films, deposited on a polycrystalline Pt substrate. A multitude of dissociation channels is observed, among which the major cation species are identified as HNCH+, HNC3H4+, C3H3+, OCNH2+, [T–OCN]+, [T–OCNH2]+, [T–O]+, and [T+H]+ and the major anions as H−, O−, CN−,and OCN−. Cation fragment desorption appears at much lower threshold energies (near 15eV) than anion fragment desorption, where the latter depends strongly on the film thickness. It is proposed that anion fragment formation and desorption results from projectile impact-induced excitation of either (1) a neutral thymine molecule, followed by fragmentation and charge exchange between the energetic neutral fragment and the substrate (or film) and/or (2) a deprotonated monoanionic thymine molecule to a dissociative state, followed by a unimolecular fragmentation of the excited thymine anion. The H− and O− fragment formations may have a further contribution from dipolar dissociation, e.g., formation of electronically excited neutral thymine, followed by dissociation into O−+[T–O]+, due to their reduced sensitivity to the film thickness. Positive-ion fragment desorption exhibits no significant dependence on film thickness before the emergence of surface charging, and originates from a kinetically assisted charge-transfer excitation. The results suggest that the potential energy of the incident ion plays a significant role in lowering the threshold energy of kinetic fragmentation of thymine. Measurements of the time-dependent film degradation yields for 100-eV Ar+ suggest a quantum efficiency for degradation of about six thymine molecules per incident ion.
Evolution of the geometrical and electronic structures of Gan(n=2–26) clusters: A density-functional theory study
Authors: Song Bin , Cao Pei-lin
Published: October 2005

Density-functional theory with generalized gradient approximation for the exchange-correlation potential has been used to calculate the lowest-energy geometries and electronic structure of neutral gallium clusters containing up to 26 atoms. Harmonic vibrational frequency analysis is undertaken to assure that the lowest-energy geometries are real local minima. With increasing cluster size, we find that the gallium clusters tend to adopt compact structures. The structures comprise triangular units that connect each other with different dihedral angles. The lowest-energy structure can be obtained by capping an atom on the structure of smaller one. The capping site occurs at a site where interactions with more atoms are available. The binding energy evolves monotonically with size, but Ga8, Ga14, and Ga20 exhibit particularly higher stability. Except Ga2 and Ga4, all even-numbered gallium clusters we studied are closed-shell singlet states with a substantial highest occupied and lowest unoccupied molecular orbitals gap. The odd-numbered clusters are open shell with a small gap. The size dependence of cluster’s ionization potentials and electron affinities is discussed and compared with available experiment.
Electron-density topology in molecular systems: Paired and unpaired densities
Authors: Lobayan Rosana M. , Bochicchio Roberto C. , Lain Luis , Torre Alicia
Published: October 2005

This work studies the partitioning of the electron density into two contributions which are interpreted as the paired and the effectively unpaired electron densities. The topological features of each density field as well as of the total density are described localizing the corresponding critical points in simple selected molecules (local formalism). The results show that unpaired electron-density concentrations occur out of the topological bonding regions whereas the paired electron densities present accumulations inside those regions. A comparison of these results with those arising from population analysis techniques (nonlocal or integrated formalisms) is reported.
Spectral broadening and diffusion by torsional motion in biphenyl
Authors: Beenken Wichard J. D. , Lischka Hans
Published: October 2005

We have studied biphenyl by time-dependent density-functional theory. In particular, we have analyzed the dependence of singlet excitation energies and transition dipoles on the torsional angle between the phenyl groups. The torsional spectrum has been computed quantum mechanically as well as semiclassically in order to understand how this influences the broadening of absorption and luminescence spectra. Our results are in best agreement with supersonic jet spectroscopy data, but also fit astonishingly well to spectra of biphenyl in condensed phase. Furthermore, we compare the torsional and vibrational relaxation and discuss qualitatively the general consequences for poly-para-phenylenes and related conjugated polymers as poly-thiophenes, considering, in particular, how side chains and solvents may affect the optical spectra.
Accelerated stochastic simulation of the stiff enzyme-substrate reaction
Authors: Cao Yang , Gillespie Daniel T. , Petzold Linda R.
Published: October 2005

The enzyme-catalyzed conversion of a substrate into a product is a common reaction motif in cellular chemical systems. In the three reactions that comprise this process, the intermediate enzyme-substrate complex is usually much more likely to decay into its original constituents than to produce a product molecule. This condition makes the reaction set mathematically “stiff.” We show here how the simulation of this stiff reaction set can be dramatically speeded up relative to the standard stochastic simulation algorithm (SSA) by using a recently introduced procedure called the slow-scale SSA. The speedup occurs because the slow-scale SSA explicitly simulates only the relatively rare conversion reactions, skipping over occurrences of the other two less interesting but much more frequent reactions. We describe, explain, and illustrate this simulation procedure for the isolated enzyme-substrate reaction set, and then we show how the procedure extends to the more typical case in which the enzyme-substrate reactions occur together with other reactions and species. Finally, we explain the connection between this slow-scale SSA approach and the Michaelis–Menten [Biochem. Z. 49, 333 (1913)] formula, which has long been used in deterministic chemical kinetics to describe the enzyme-substrate reaction.
Second-harmonic generation of solvated molecules using multiconfigurational self-consistent-field quadratic response theory and the polarizable continuum model
Authors: Frediani Luca , Ågren Hans , Ferrighi Lara , Ruud Kenneth
Published: October 2005

We present the first implementation of the quadratic response function for multiconfigurational self-consistent-field wave functions of solvated molecules described by a polarizable continuum model employing a molecule-shaped cavity. We apply the methodology to the first hyperpolarizability β and, in particular, the second-harmonic generation process for a series of conjugated push-pull oligomers, as well as for para-nitroaniline. The effect of solvation on the dispersion of the hyperpolarizability and the change in the hyperpolarizability for increasing chain length of the oligomers in vacuum and in solution is considered. The effect of a correlated description is analyzed by comparing the Hartree-Fock hyperpolarizabilities to the multiconfigurational self-consistent-field hyperpolarizabilities. The effect of geometry relaxation in the solvent on the properties of the solvated molecules are also investigated.
Accurately solving the electronic Schrödinger equation of small atoms and molecules using explicitly correlated (r12-)MR-CI. VIII. Valence excited states of methylene (CH2)
Authors: Flores Jesús R. , Gdanitz Robert J.
Published: October 2005

We compute the adiabatic transition energies of methylene (CH2) from the ground state to the lowest electronically excited valence states using the r12-MR-ACPF-2 method with a large basis set and an extended reference space. We recall that this method aims at reaching the basis-set and full configuration interaction (CI) limits simultaneously. Our best excitation energies, Te (T0), are 9.22 (8.87) (ãA11, corrected for relativistic and adiabatic effects), 31.98 (31.86) (b̃B11), and 57.62 (57.18)kcalmol−1 (c̃A11) (both uncorrected). We are able to reach the respective basis-set limits that closely that the remaining errors of our (uncorrected) calculations are clearly due to the MR-ACPF-2 method. While we are unable to assess the error of the latter method in a systematic way, we still believe that it is rather unlikely that the errors of our excitation energies exceed ±0.10kcalmol−1. We finally observe that our (corrected) ã state values deviate by only −0.10 (−0.10)kcalmol−1 from the results of Császár et al. [J. Chem. Phys. 118, 10631 (2003)] — who did careful extrapolations to the valence full-CI and basis-set limits and added a correction for the core correlation — and that the deviation from experiment is only −0.13 (−0.13)kcalmol−1. From these excellent agreements we conclude that our excitation energies to the b̃ and c̃ states are similarly accurate.
Thermodynamic model of quasiliquid formation on H2O ice: Comparison with experiment
Authors: Henson B. F. , Voss L. F. , Wilson Kevin R. , Robinson J. M.
Published: October 2005

We have developed a new thermodynamic theory of the quasiliquid layer, which has been shown to be effective in modeling the phenomenon in a number of molecular systems. Here we extend our analysis to H2O ice, which has obvious implications for environmental and atmospheric chemistry. In the model, the liquid layer exists in contact with an ice defined as a two-dimensional lattice of sites. The system free energy is defined by the bulk free energies of ice Ih and liquid water and is minimized in the grand canonical ensemble. An additional configurational entropy term arises from the occupation of the lattice sites. Furthermore, the theory predicts that the layer thickness as a function of temperature depends only on the liquid activity. Two additional models are derived, where slightly different approximations are used to define the free energy. With these two models, we illustrate the connection between the quasiliquid phenomenon and multilayer adsorption and the possibility of a two-dimensional phase transition connecting a dilute low coverage phase of adsorbed H2O and the quasiliquid phase. The model predictions are in agreement with a subset of the total suite of experimental measurements of the liquid thickness on H2O ice as a function of temperature. The theory indicates that the quasiliquid layer is actually equivalent to normal liquid water, and we discuss the impact of such an identification. In particular, observations of the liquid layer to temperatures as low as 200K indicate the possibility that the quasiliquid is, in fact, an example of deeply supercooled normal water. Finally, we briefly discuss the obvious extension of the pure liquid theory to a thermodynamic theory of interfacial solutions on ice in the environment.
Effect of “inverse melting transition” of aqueous triblock copolymer solutions on solute rotational dynamics
Authors: Mali K. S. , Dutt G. B. , Ganguly R. , Mukherjee T.
Published: October 2005

Rotational dynamics of two structurally similar hydrophobic solutes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been investigated in 30% w∕v aqueous solution of triblock copolymer, poly(ethyleneoxide)20-poly(propyleneoxide)70-poly(ethyleneoxide)20 as a function of temperature. This study has been undertaken in an attempt to explore how the dynamics of a solute molecule solubilized in a copolymer solution is influenced when it undergoes sol-to-gel transition. It has been observed that the anisotropy decays of both DMDPP and DPP can be described by biexponential functions in the sol as well as in the gel phase. This observation has been rationalized on the basis of the probe molecule undergoing two different kinds of motion rather than being located in two different regions of the micelle. Even in the gel phase, which results as a consequence of micelle-micelle entanglement due to an increase in their volume fraction, the rotational relaxation of the solutes is similar to that observed in the micellar solution. The outcome of this work indicates that even though these gels have very high macroscopic viscosities and hence do not flow, the microenvironments experienced by the solutes are akin to that of a micellar solution.
Reducing the effects of adhesion and friction in microelectromechanical systems (MEMSs) through surface roughening: Comparison between theory and experiments
Authors: Tayebi Noureddine , Polycarpou Andreas A.
Published: October 2005

Due to the large surface-to-volume ratios and the low loads encountered in microelectromechanical systems (MEMSs), the surface forces become important and may lead to permanent adhesion and high friction between near contacting and contacting surfaces. The effect of these forces can be reduced through surface texturing (roughening) at the contact interface. Moreover, modifying the distribution of the contacting surface asperities so that it becomes positively asymmetric (unbalance between the peak and valley heights) and as peaky as possible (making slender asperities) reduces these forces even further. In the current study, the effects of these parameters, i.e., roughness, asymmetry, and peakiness, on reducing the adhesion and friction in polycrystalline silicon (also referred as polysilicon) MEMS surfaces, were theoretically and experimentally investigated. Polysilicon films with different levels of roughness, asymmetry, and peakiness were fabricated. The roughness characteristics of these films were used in a continuum-based micromechanics model to predict the level of adhesion and friction in actual MEMS devices. Experiments were also conducted to evaluate the adhesion pull-off force and friction coefficient associated with these films. It is found, both experimentally and theoretically, that the adhesion pull-off force and friction coefficient can be reduced by an order of magnitude by increasing the roughness, asymmetry, and peakiness of the contacting surfaces under low external normal forces. Stick-slip behavior, which may be indicative of the presence of adhesive forces, also reduces considerably with the increase of these parameters. Lastly, good agreement was found between simulations and experimental results. Thus, such a model could be used to determine the critical characteristics of a microstructure prior to fabrication to prevent adhesion and lower friction in terms of surface roughness, mechanical properties, and environmental conditions.
Solid-state optical properties of linear polyconjugated molecules: π-stack contra herringbone
Authors: Gierschner Johannes , Ehni Markus , Egelhaaf Hans-Joachim , Milián Medina Begoña , Beljonne David , Benmansour Hadjar , Bazan Guillermo C.
Published: October 2005

The intermolecular arrangement in the solid state and the consequences on the optical and photophysical properties are studied on different derivatives of oligophenylenevinylenes by UV/VIS absorption and angular-resolved polarized fluorescence spectroscopy. Unsubstituted distyrylbenzene (DSB) organizes in a herringbone manner, with the long axes of the molecules oriented in parallel, but the short axes almost perpendicular to each other. Fluorinated distyrylbenzene (F12DSB) as well as the DSB:F12DSB cocrystals prefer cofacial π-stacking in the solid state. For all structures, the consequence of the parallel alignment of the transition moments is a strongly blueshifted H-type absorption spectrum and a low radiative rate constant kF. Significant differences are observed for the emission spectra: the perpendicular arrangement of the short axes in DSB crystals leads to only very weak intermolecular vibronic coupling. Hence the emission spectrum is well structured, very similar to the one in solution. For F12DSB and DSB:F12DSB, the cofacial arrangement of the adjacent molecules enables strong intermolecular vibronic coupling of adjacent molecules. Thus, an unstructured and strongly redshifted excimerlike emission spectrum is observed. The differences in the electronic nature of the excited states are highlighted by quantum-chemical calculations, revealing the contribution of interchain excitations to the electronic transitions.
Optoelectronic properties of polymer-nanocrystal composites active at near-infrared wavelengths
Authors: Solomeshch Olga , Kigel Ariel , Saschiuk Aldona , Medvedev Vlad , Aharoni Assaf , Razin Alexey , Eichen Yoav , Banin Uri , Lifshitz Efrat , Tessler Nir
Published: October 2005

We report a systematic study of the optoelectronic processes occurring in composites made of near-infrared (IR) emitting nanocrystals and conjugated polymers. We focus on PbSe and InAs∕ZnSe blended with polyphenylenevinylene-type polymers. We find that the process responsible for quenching the visible luminescence of the polymer by the nanocrystal varies depending on the nanocrystal composite. Moreover, the high (66%) energy-transfer efficiency from the polymer to the PbSe nanocrystal does result in significant emission at the near IR. Our measurements suggest that the host may be doping the PbSe nanocrystal, thus making the nonradiative Auger process favorable. For InAs we find the energy levels well aligned inside the polymer band gap, making it an efficient charge trap which acts as a luminescence center. Through two-dimensional numerical modeling of the charge transport in such composite films we highlight the importance of morphology (nanocrystal distribution) control.
Characterization of crack-free relaxed GaN grown on 2″ sapphire
Authors: Kasic A. , Gogova D. , Larsson H. , Ivanov I. , Hemmingsson C. , Yakimova R. , Monemar B. , Heuken M.
Published: October 2005

We demonstrate the growth of high-quality and virtually strain-free bulklike GaN by hydride vapor-phase epitaxy in a vertical atmospheric-pressure reactor with a bottom-fed design. The 300-μm-thick GaN layer was grown on a 2″ (0 0 0 1) sapphire substrate buffered with a ∼2-μm-thick GaN layer grown by metal-organic chemical-vapor deposition. During the cool down process to room temperature, cracking was induced in the sapphire substrate, thereby allowing the bulklike GaN layer to relax without provoking cracking of itself. The crystalline quality and the residual strain in the 2″ GaN wafer were investigated by various characterization techniques. The lateral homogeneity of the wafer was monitored by low-temperature photoluminescence mapping. High-resolution x-ray diffraction and photoluminescence measurements proved the high crystalline quality of the material grown. The position of the main near-band-gap photoluminescence line and the phonon spectra obtained from infrared spectroscopic ellipsometry show consistently that the 2″ crack-free GaN is virtually strain-free over a diameter of approximately 4cm.
Millimeter-wave-detected, millimeter-wave optical polarization spectroscopy
Authors: Steeves Adam H. , Bechtel Hans A. , Coy Stephen L. , Field Robert W.
Published: October 2005

We report a new form of microwave optical double-resonance spectroscopy called millimeter-wave-detected, millimeter-wave optical polarization spectroscopy (mmOPS). In contrast to other forms of polarization spectroscopy, in which the polarization rotation of optical beams is detected, the mmOPS technique is based on the polarization rotation of millimeter waves induced by the anisotropy from optical pumping out of the lower or upper levels of the millimeter wave transition. By monitoring ground-state rotational transitions with the millimeter waves, the mmOPS technique is capable of identifying weak or otherwise difficult-to-observe optical transitions in complex chemical environments, where multiple molecular species or vibrational states can lead to spectral congestion. Once a transition is identified, mmOPS can then be used to record pure rotational transitions in vibrationally and electronically excited states, with the resolution limited only by the radiative decay rate. Here, the sensitivity of this nearly-background-free technique is demonstrated by optically pumping the weak, nominally spin-forbidden CS eΣ−3−XΣ+1(2-0) and dΔ3−XΣ+1(6-0) electronic transitions while probing the CS XΣ+1(v″=0,J″=2-1) rotational transition with millimeter waves. The J′=2,N′=2←J′=1,N′=1 pure rotational transition of the CS eΣ−3(v′=2) state is then recorded by optically preparing the J′=1,N′=1 level of the eΣ−3(v′=2) state via the J′=1,N′=1←J″=1 transition of the eΣ−3−XΣ+1(2-0) band.
Conditional filtering method for large-eddy simulation of turbulent nonpremixed combustion
Authors: Kim Seung Hyun , Pitsch Heinz
Published: October 2005

The conditional filtering method is proposed as a subfilter combustion model for large-eddy simulation (LES) of turbulent nonpremixed combustion. The novel method is based on conditional filtering of a reactive scalar field and an extension of conditional moment closure (CMC) for LES. Filtering conditioned on isosurfaces of the mixture fraction is adopted to resolve small-scale mixing and chemical reactions in nonpremixed combustion. The conditionally filtered equations are derived and the closure assumptions are discussed. A priori tests are performed using direct numerical simulation data for reacting mixing layers. The primary closure assumption on the subfilter flux in mixture fraction space is shown to work much better than the corresponding closure for the Reynolds averaged CMC due to resolved large-scale fluctuations of the scalar dissipation rate and of reactive scalars. Results show that first-order closure of the reaction rate performs well except for the boundaries of flame holes. In the boundaries of flame holes, fluctuations of reactive scalars around the conditionally filtered values are large enough for the effects of higher-order correlations to be significant. The accuracy of the first-order closure is less sensitive to the level of local extinction than that of first-order CMC, since large-scale fluctuations of reactive scalars on isosurfaces of the mixture fraction are resolved. This shows that extinction processes occur primarily over length scales comparable to the large scales of the turbulence. The integrated conditional filtering approach is introduced to reduce the computational cost and to resolve the low probability problem in the conditional filtering method. While the assumption of homogeneity in the integration direction is not as good as in the conditional average, the integrated formulation is shown to represent the extinction process caused by large-scale fluctuations of the scalar dissipation rate quite well.
Magnetically accelerated, ultrahigh velocity flyer plates for shock wave experiments
Authors: Lemke R. W. , Knudson M. D. , Bliss D. E. , Cochrane K. , Davis J.-P. , Giunta A. A. , Harjes H. C. , Slutz S. A.
Published: October 2005

The intense magnetic field produced by the 20 MA Z accelerator is used as an impulsive pressure source to accelerate metal flyer plates to high velocity for the purpose of performing plate impact, shock wave experiments. This capability has been significantly enhanced by the recently developed pulse shaping capability of Z, which enables tailoring the rise time to peak current for a specific material and drive pressure to avoid shock formation within the flyer plate during acceleration. Consequently, full advantage can be taken of the available current to achieve the maximum possible magnetic drive pressure. In this way, peak magnetic drive pressures up to 490 GPa have been produced, which shocklessly accelerated 850μm aluminum (6061-T6) flyer plates to peak velocities of 34km∕s. We discuss magnetohydrodynamic (MHD) simulations that are used to optimize the magnetic pressure for a given flyer load and to determine the shape of the current rise time that precludes shock formation within the flyer during acceleration to peak velocity. In addition, we present results pertaining to plate impact, shock wave experiments in which the aluminum flyer plates were magnetically accelerated across a vacuum gap and impacted z-cut, α-quartz targets. Accurate measurements of resulting quartz shock velocities are presented and analyzed through high-fidelity MHD simulations enhanced using optimization techniques. Results show that a fraction of the flyer remains at solid density at impact, that the fraction of material at solid density decreases with increasing magnetic pressure, and that the observed abrupt decrease in the quartz shock velocity is well correlated with the melt transition in the aluminum flyer.
Contributions to in-plane piezoresponse on axially symmetrical samples
Authors: Peter F. , Rüdiger A. , Waser R. , Szot K. , Reichenberg B.
Published: October 2005

We report on the influence of system-immanent asymmetries on the interpretation of in-plane piezoresponse force microscopy (PFM). As PFM is a surface scanning method, the electromechanical interaction of probe tip and sample is a key aspect of all experiments. An initial characterization of topography, surface state, and conductivity is mandatory to separate their signal from the response due to an in-plane polarization state. Our findings underline that any reduction of radial symmetry in the tip-sample system creates an otherwise symmetry-prohibited in-plane signal.