bluuebunny/crossref_metadata_2025 huggingface dataset:
Heliomagnetic field near Earth, 1428–2005
Authors: McCracken K. G.
Published: September 2007

The cosmic ray record for the interval 1428–2005 has been inverted using the method of Caballero‐Lopez et al. (2004) to estimate the annual averages of the heliomagnetic field (HMF) near Earth. There is good agreement with the results obtained by others using two independent methodologies based upon the sunspot and geomagnetic records. This provides confidence in the overall validity of all three methods and supports the use of the cosmogenic record to investigate the HMF prior to the commencement of the sunspot record. There is disagreement with another method based on the geomagnetic record that remains to be resolved. Throughout the ∼580‐year interval studied here, the estimated HMF increased by a factor of ∼4.5 from an 11‐year average of ∼1.5 nT during the Spoerer Minimum (1420–1540) to ∼7 nT since 1950. Between the several Grand Minima, it exhibits increasing “floor” values similar to the lower asymptote of ∼5.2 nT evident in satellite data since 1965. The cosmogenic cosmic ray data exhibit a ∼2300‐year periodicity, and it is proposed that the steadily increasing HMF since the 15th century represents the first quarter cycle of an associated ∼2300‐year periodicity in the HMF. Recent theoretical studies of the origin of the open magnetic field are used to infer that the total magnetic flux of the Sun has increased by a factor of ∼4.5 since the Spoerer Minimum and might increase further over the next ∼500 years. In more recent time, the sunspot minimum estimates for the Dalton minimum (1810–1820) and Gleissberg minimum (1889–1901) are ∼2.5 nT, rising to ∼4 nT in the vicinity of 1850. The amplitude of the 11‐year cycle throughout the Gleissberg cycle (1810–1900) is estimated to be ∼2 nT. The estimated field is ∼3.5 nT for each of the sunspot minima 1911–1944, increasing in a stepwise fashion to ∼5.2 nT between the minima of 1944 and 1954. The cosmic ray and satellite data indicate that the field strength then approximated 5.2 nT for the four sunspot minima, 1965–1996.
Near‐field response to transpression along the southern San Andreas fault, based on exhumation of the northern San Gabriel Mountains, southern California
Authors: Buscher Jamie T. , Spotila James A.
Published: September 2007

Near‐field mountain building is common along transpressive faults, but how vertical deformation relates to oblique plate motion versus local structural complexity has not been universally established. We have investigated the mechanisms of exhumation along the southern San Andreas fault in the northern San Gabriel Mountains to examine the effects of pure‐shear‐dominated transpression (obliquity >20°). Although rugged topography suggests near‐field uplift, low‐temperature thermochronometry indicates minimal exhumation. Apatite (U‐Th)/He ages are >10 Ma along narrow ridges within the fault zone, implying near‐field rock uplift is not pervasive along this oblique plate boundary. Ages farther from the fault are younger (∼5 Ma) and suggest a deformation event occurred during the transition of activity from the San Gabriel fault to the modern San Andreas strand. These results reveal a heterogeneous crustal response to transpression and imply that secondary structures or geometrical complexities play a primary role in near‐field bedrock uplift along transpressive faults.
What do the HIRM and S‐ratio really measure in environmental magnetism?
Authors: Liu Qingsong , Roberts Andrew P. , Torrent José , Horng Chorng‐Shern , Larrasoaña Juan C.
Published: September 2007

The “hard” isothermal remanent magnetization (HIRM) and the S‐ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations, respectively, of antiferromagnetic minerals (hematite and goethite) in mineral mixtures. We demonstrate that synthetic Al‐substituted hematite and goethite exhibit a wide range of coercivities, which significantly influences the HIRM and S‐ratio. These parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite/goethite. To circumvent this problem, we propose a new parameter (the L‐ratio), which is the ratio of two remanences after alternating field (AF) demagnetization of an IRM imparted in a 1 T field with a peak AF of 100 mT and 300 mT: IRMAF@300mT/IRMAF@100mT. These parameters are easily measured using modern vibrating sample or alternating gradient magnetometers. Changes in HIRM only reflect changes in the absolute concentration of hematite and/or goethite if the L‐ratio is relatively constant. Conversely, L‐ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite/goethite. Corresponding HIRM and S‐ratio variations should be interpreted with caution in such cases. The L‐ratio can be determined using equivalent terms depending on available instrumentation and measurement protocols. For example, the HIRM is equivalent to IRMAF@300mT. Likewise, 0.5*(SIRM + IRM−100mT), where IRM−100mT represents the remanent magnetization obtained by first saturating the sample in a high field and then applying a back‐field of −100 mT, is equivalent to IRMAF@100mT. The HIRM/[0.5*(SIRM + IRM−100mT)] ratio is therefore a suitable substitute for the L‐ratio when measurements are made with a long‐core magnetometer. The newly proposed L‐ratio is straightforward to measure on a wide range of instruments and can provide significant new insights and reduce ambiguities associated with interpretation of two widely used parameters in environmental magnetism, the HIRM and S‐ratio.
Relative contributions of biomass burning emissions and atmospheric transport to carbon monoxide interannual variability
Authors: Szopa S. , Hauglustaine D. A. , Ciais P.
Published: September 2007

Biomass burning is an important source of greenhouse gases and of ozone precursors such as CO. Recent analyses combining the use of satellite data and of ecosystem models showed that wild fire emissions are subject to a large interannual variability. In this study, NOAA/ESRL CO surface measurements are used in conjunction with the results from the LMDz‐INCA global chemistry‐transport model and analyzed over the 1997–2001 period in order to quantify the relative contributions of biomass burning emissions and of climate to the CO interannual variability (IAV). Over the considered five year period, the mean CO IAV was found to be about 11% for stations far from regional pollution and 4.5% for Antarctic stations. At both southern and northern high latitude stations, the CO IAV is controlled almost equally by variations in biomass burning emissions and atmospheric meteorology. On the contrary, meteorological variability prevails in the tropics, where it explains 50% to 90% of the CO IAV. Variability in long‐range transport and climate is thus the dominant process controlling the CO interannual variability, except during specific episodes, such as the intense fires associated with the 1997–1998 El Niño event.
What drives magnetospheric activity under northward IMF conditions?
Authors: Pulkkinen T. I. , Palmroth M. , McPherron R. L.
Published: September 2007

We show that the dependence of magnetospheric activity on the driving solar wind electric field is qualitatively different for northward and southward IMF conditions. For southward IMF, the AE‐index is driven by the solar wind electric field, but for a given EY, AE is higher when the solar wind speed is higher. For northward IMF, AE does not scale with the electric field, but is lowest for low solar wind speed and small IMF clock‐angles. These results are explained by a quantitative analysis of the amount of energy that enters the magnetosphere using the GUMICS‐4 global MHD simulation.
Barotropic tides of the Southern Indian Ocean and the Amery Ice Shelf cavity
Authors: Maraldi Claire , Galton‐Fenzi Benjamin , Lyard Florent , Testut Laurent , Coleman Richard
Published: September 2007

The 8 main tidal constituents were computed using a finite element, hydrodynamic ocean tide model over the South Indian Ocean region. The discretization of the domain is of the order of 100 km over the deep ocean and a few hundred meters near the coast. Such refinement in the grid resolution enables wave propagation and damping on the continental shelves to be solved correctly. The model used the GEBCO 1‐minute global bathymetric grid which was improved with updated topographic data. The model solutions show good agreement with in‐situ observations and Topex‐Poseidon altimeter measurements and are significantly better than previously published solutions. We obtain a combined standard deviation of 1.4 cm for differences of our new regional model against independent observations compared to about 2.5 cm for the other tide models. The greatest improvements are found around the Kerguelen Islands, around Antarctica and beneath the Amery Ice Shelf and can be explained by the high grid resolution used and the particular attention given to the accuracy of the bathymetry in those regions.
Influence of a dust storm on carbon monoxide and water vapor over the Indo‐Gangetic Plains
Authors: Bhattacharjee Partha S. , Prasad Anup K. , Kafatos Menas , Singh Ramesh P.
Published: September 2007

Dust storms are meteorological phenomena that produce air quality hazards over specific regions lasting from a few hours to many days. They are common in the western part of India during the months of April–June, particularly over Delhi and the surrounding Indo‐Gangetic (IG) plains. In this paper, a dust storm event over Delhi, Kanpur and Varanasi during 1–11 April 2005 was used to study the vertical changes in the atmosphere. We studied data from the Measurement of Pollution in the Atmosphere (MOPITT) instrument onboard the Terra satellite, daytime vertical carbon monoxide (CO) mixing ratio and the Atmospheric Infra‐red Sounder (AIRS) onboard the Aqua satellite water vapor mass mixing ratio associated with dust storm over three locations in the IG plains. Evidence of vertical transport of CO to upper troposphere (UT) is observed from stability indices derived from radiosonde data, NCEP reanalysis wind and HYSPLIT model over Delhi. The strong upward convection during dust storms reduces CO in the lower troposphere and increases CO at around 350 hPa pressure level. Water vapor mass mixing ratio shows an increase at 700–850 hPa pressure level during the dust storm event over all locations. The changes of water vapor mass mixing ratio and CO during the dust storm are found to be more pronounced over Delhi and Kanpur while they are comparatively less over Varanasi, due to low intensity during transport of dust from west to east in the IG plains. The increased concentration of CO and water vapor mass mixing ratio at different pressure levels 350 hPa and 700–850 hPa respectively with corresponding decrease in surface concentration (at pressure level 700–1000 hPa and 925–1000 hPa respectively) have been investigated during a major dust storm period. The changes in CO and water vapor mass mixing ratio are found to be consistent with the observed changes in vertical stability of the atmosphere.
Comparative study of short‐term diurnal tidal variability
Authors: Liu H.‐L. , Li T. , She C.‐Y. , Oberheide J. , Wu Q. , Hagan M. E. , Xu J. , Roble R. G. , Mlynczak M. G. , Russell J. M.
Published: September 2007

The wind and temperature measurements from an unusually long period operation of the sodium lidar at Colorado State University (41°N, 105°W) around September equinox 2003 showed significant short‐term tidal variability. Coincident with the large tidal changes, a strong temperature inversion layer was also observed above 90 km. Examination of the simultaneous temperature measurement from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument, on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite, not only confirms the existence of the inversion layer but also reveals the global nature of the inversion, suggesting the presence of a transient planetary wave in the mesosphere. The large tidal variability, therefore, is probably a consequence of the interaction between the transient planetary wave and tides. This possibility is investigated by using the NCAR thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) and by comparing model results with the lidar, SABER, and TIMED Doppler Interferometer (TIDI) measurements. With a large transient planetary wave specified at the model lower boundary, the model is able to produce strong diurnal tidal variability comparable to that from the lidar observation, and the modeled temperature inversion is similar to that from the SABER measurement. The model results suggest that the planetary/tidal wave interaction excites nonmigrating tides and modulates the gravity modes and/or the rotational modes of the diurnal migrating tide. Among the nonmigrating tides, the diurnal zonally symmetric (S = 0) component is the strongest, and its interaction with the planetary wave leads to a strong diurnal eastward wave number 1 component.
Influence of the North Atlantic Oscillation on water resources in central Iberia: Precipitation, streamflow anomalies, and reservoir management strategies
Authors: López‐Moreno Juan I. , Beguería Santiago , Vicente‐Serrano Sergio M. , García‐Ruiz José M.
Published: September 2007

This paper analyzes the influence of the extreme phases of the winter North Atlantic Oscillation (NAO) on water resources in the Spanish region of the Tagus River basin. By analyzing a winter NAO index based on station sea level pressure, the years between 1957 and 2003 were classified as normal, positive, and negative NAO years. A statistical test was then applied to monthly data series of precipitation, river discharge, reservoir storage, and reservoir release to analyze the variations in these variables. For all four variables, significant differences were found between positive and negative NAO years, the former resulting in reduced water availability (negative anomalies) and the latter resulting in increased water availability (positive anomalies). The influence of extreme NAO winters was found to act with different time lags on different variables: The effect of extreme NAO winters on precipitation was found to be quite immediate (and significant for December to March), but this effect was observed later in the year and lasted longer for river discharge, reservoir storage, and water release. Positive and negative NAO years were also found to have different effects on these variables, in that the effects of positive years were more sustained and those of negative years were more rapid and less prolonged. In spite of the high variability of the availability of water resources, the strategies for management of the reservoir system of the basin were found in most cases to provide a regular supply that meets water demands. However, our results also indicate that these water management practices are not adequate for the expected scenarios of climate change and increasing water demand.
The biogeographical distribution of closely related freshwater sediment bacteria is determined by environmental selection
Authors: Gray Neil D , Brown Angela , Nelson Darryl R , Pickup Roger W , Rowan Arlene K , Head Ian M
Published: September 2007

The role of environmental selection in governing the structure of communities of freshwater sulfur bacteria (Achromatium spp) was experimentally tested by mixing sediments from two geographically separated lakes (Rydal Water (RY) and Hell Kettles (HK)) that harboured Achromatium spp. Community profiles of Achromatium spp in sediment microcosms at day 0 and after 60 days were compared to determine whether initial Achromatium community composition or subsequent selection by the sediment environment had greater influence in dictating the final Achromatium community structure. It was found that Achromatium spp from the HK community became established in mixed sediments at the expense of members of the RY community. This selection for the HK Achromatium community was more pronounced when sediment composition was manipulated to resemble HK sediments. Our findings definitively demonstrate that environmental selection is the primary determinant of Achromatium community structure in these sediments.
Comparison of Language Skills of Adjudicated and Nonadjudicated Adolescent Males and Females
Authors: Blanton Debra J. , Dagenais Paul A.
Published: September 2007

Purpose

This study attempted to determine whether there were any differences or similarities in the basic cognitive and language skills of 4 groups of adolescents: adjudicated (a judge’s decision to establish responsibility for a delinquent act) and nonadjudicated male and females.

Method

The 4 groups of adolescents were 18 adjudicated females, 18 nonadjudicated females, 14 adjudicated males, and 14 nonadjudicated males. They were evaluated using the Kaufman Brief Intelligence Test (K–BIT; A. S. Kaufman & N. L. Kaufman, 1990) as a screener for intelligence and the Clinical Evaluation of Language Fundamentals—3 (CELF–3; E. Semel, E. Wiig, & W. Secord, 1995) as a screener for language skills.

Results

No gender differences were found between the groups for either the K–BIT or the CELF–3. All groups scored within normal limits on the K–BIT subtests, but lower on the vocabulary portion of the K–BIT compared to the matrices (nonverbal) portion. There were differences found between adjudicated and nonadjudicated groups on the CELF–3, with the adjudicated groups having lower scores.

Implications

As the majority of adjudicated adolescents had not been previously identified as having difficulties with language abilities, this population could benefit from early assessment. Also, children who are at risk for incarceration should be screened for possible language deficits that could contribute to the many difficulties experienced by adolescents.
Electroacoustic Evaluation of Frequency-Modulated Receivers Interfaced With Personal Hearing Aids
Authors: Schafer Erin C. , Thibodeau Linda M. , Whalen Holly S. , Overson Gary J.
Published: September 2007

Purpose

The purpose of this study was to compare the electroacoustic outputs of frequency-modulated (FM) systems coupled to hearing aids.

Method

Electroacoustic performance of FM systems coupled to hearing aids was determined for 3 FM receivers: body-worn with neck loop, ear-level nonprogrammable, and ear-level programmable. Systems were evaluated using the FM-advantage approach suggested by the American Speech-Language-Hearing Association’s “Guidelines for Fitting and Monitoring FM Systems” (2002). Output differences between the hearing aid and FM system were examined for typical input levels with a complex signal to verify an FM advantage and for high-input levels with a pure-tone signal to confirm similarity in maximum output.

Results

Measurements from the FM receivers with neck loops showed significant low-frequency reduction and the most variable performance of the 3 types of systems. Less variable measurements were obtained with the ear-level FM receivers, and programmable FM receivers allowed for the most flexibility in obtaining an FM advantage. Findings of variability may be related to the type of system and the compression characteristics of the devices.

Conclusion

Findings of great variability in electroacoustic performance of all systems support the need for electroacoustic evaluation, particularly when observing the effects of various settings on new technology.
Coarse-grained force field for simulating polymer-tethered silsesquioxane self-assembly in solution
Authors: Chan Elaine R. , Striolo Alberto , McCabe Clare , Cummings Peter T. , Glotzer Sharon C.
Published: September 2007

A coarse-grained model has been developed for simulating the self-assembly of nonyl-tethered polyhedral oligomeric silsesquioxane (POSS) nanoparticles in solution. A mapping scheme for groups of atoms in the atomistic molecule onto beads in the coarse-grained model was established. The coarse-grained force field consists of solvent-mediated effective interaction potentials that were derived via a structural-based coarse-graining numerical iteration scheme. The force field was obtained from initial guesses that were refined through two different iteration algorithms. The coarse-graining scheme was validated by comparing the aggregation of POSS molecules observed in simulations of the coarse-grained model to that observed in all-atom simulations containing explicit solvent. At 300K the effective coarse-grained potentials obtained from different initial guesses are comparable to each other. At 400K the differences between the force fields obtained from different initial guesses, although small, are noticeable. The use of a different iteration algorithm employing identical initial guesses resulted in the same overall effective potentials for bare cube corner bead sites. In both the coarse-grained and all-atom simulations, small aggregates of POSS molecules were observed with similar local packings of the silsesquioxane cages and tether conformations. The coarse-grained model afforded a savings in computing time of roughly two orders of magnitude. Further comparisons were made between the coarse-grained monotethered POSS model developed here and a minimal model developed in earlier work. The results suggest that the interactions between POSS cages are long ranged and are captured by the coarse-grained model developed here. The minimal model is suitable for capturing the local intermolecular packing of POSS cubes at short separation distances.
Influence of hydrogen absorption on structural and electrical transport properties of Ho1−xMmxCo2 alloys
Authors: Srinivas G. , Sankaranarayanan V. , Ramaprabhu S.
Published: September 2007

The structural and electrical transport properties of Ho1−xMmxCo2 (x=0, 0.1, 0.2, 0.3, and 0.4 and Mm=mischmetal) alloys and their hydrides in the hydrogen concentration (y) range of 0⩽y⩽3.6 have been determined through the powder x-ray diffraction (XRD) and temperature dependence of electrical resistivity [ρ(T)] measurements. At room temperature, these compounds crystallize in MgCu2-type (C15) structure. The crystalline nature and lattice expansion of hydrogenated alloys have been studied using the hydrogen concentration dependence of XRD peak analysis indicating the coexistence of two hydride phases at intermediate hydrogen concentrations. The temperature dependence of the electrical resistivity of alloys has been discussed based on the conduction electron scattering and spin fluctuation scattering mechanisms. The changes in magnetic ordering and transition temperature upon Mm substitution and at different concentrations of hydrogen loadings have been discussed. Furthermore, the transformation from metalliclike conductivity to thermally activated conduction mechanism and different conduction regimes of temperature dependent resistivity upon increasing H concentration have been well described by Kondo-type and electron-electron scattering effects.
Examination of a critical roughness height for outer layer similarity
Authors: Flack K. A. , Schultz M. P. , Connelly J. S.
Published: September 2007

The existence of a critical roughness height for outer layer similarity between smooth and rough wall turbulent boundary layers is investigated. Results are presented for boundary layer measurements on flat plates covered with sandgrain and woven mesh with the ratio of the boundary layer thickness to roughness height (δ∕k) varying from 16 to 110 at Reθ=7.3×103–13×103. In all cases tested, the layer directly modified by the roughness (the roughness sublayer) is confined to a region <5k or <3ks from the wall (where ks is the equivalent sandgrain roughness height). In the larger roughness cases, this region of turbulence modification extends into the outer flow. However, beyond 5k or 3ks from the wall, similarity in the turbulence quantities is observed between the smooth and rough wall boundary layers. These results indicate that a critical roughness height, where the roughness begins to affect most or all of the boundary layer, does not exist. Instead, the outer flow is only gradually modified with increasing roughness height as the roughness sublayer begins to occupy an ever increasing fraction of the outer layer.
Contribution of rotational diffusion to pulsed field gradient diffusion measurements
Authors: Baldwin Andrew J. , Christodoulou John , Barker Paul D. , Dobson Christopher M. , Lippens Guy
Published: September 2007

NMR diffusion experiments employing pulsed field gradients are well established as sensitive probes of the displacement of individual nuclear spins in a sample. Conventionally such measurements are used as a measure of translational diffusion, but here we demonstrate that under certain conditions rotational motion will contribute very significantly to the experimental data. This situation occurs when at least one spatial dimension of the species under study exceeds the root mean square displacement associated with translational diffusion, and leads to anomalously large apparent diffusion coefficients when conventional analytical procedures are employed. We show that in such a situation the effective diffusion coefficient is a function of the duration of the diffusion delay used, and that this dependence provides a means of characterizing the dimensions of the species under investigation.
Dust void formation in complex plasmas with two various sizes of dust grains
Authors: Liu Yue , Song Yan , Wang Zheng-Xiong , Wang Xiaogang
Published: September 2007

A self-consistent nonlinear model for dust void is employed to study the void evolution in cylindrical complex plasmas with two various sizes of dust grains. It is shown that the system reaches a steady state approximately with the smaller dust grains form the void structure, and the bigger dust grains keep moving outwards. The time evolution of the velocity and density of both sizes of the dust grains is plotted and the corresponding physical mechanism is discussed.
Instability of a viscous interface under horizontal oscillation
Authors: Talib Emma , Juel Anne
Published: September 2007

The linear stability of superposed layers of viscous, immiscible fluids of different densities subject to horizontal oscillations, is analyzed with a spectral collocation method and Floquet theory. We focus on counterflowing layers, which arise when the horizontal volume-flux is conserved, resulting in a streamwise pressure gradient. This model has been shown to accurately predict the onset of the frozen wave observed experimentally [E. Talib, S. V. Jalikop, and A. Juel, J. Fluid Mech. 584, 45 (2007)]. The numerical method enables us to gain new insights into the Kelvin–Helmholtz (KH) mode usually associated with the frozen wave, and the harmonic modes of the parametric-resonant instability, by resolving the flow for an exhaustive range of vibrational to viscous forces ratios and viscosity contrasts. We show that the viscous model is essential to accurately predict the onset of each mode of instability. We characterize the evolution of the neutral curves from the multiple modes of the parametric-resonant instability to the single frozen wave mode encountered in the limit of practical flows. We find that either the KH or the first resonant mode may persist when the fluid parameters are varied toward this limit. Interestingly, these two modes exhibit opposite dependencies on the viscosity contrast, which are understood by examining the eigenmodes near the interface.
Buoyancy-driven convection around chemical fronts traveling in covered horizontal solution layers
Authors: Rongy L. , Goyal N. , Meiburg E. , De Wit A.
Published: September 2007

Density differences across an autocatalytic chemical front traveling horizontally in covered thin layers of solution trigger hydrodynamic flows which can alter the concentration profile. We theoretically investigate the spatiotemporal evolution and asymptotic dynamics resulting from such an interplay between isothermal chemical reactions, diffusion, and buoyancy-driven convection. The studied model couples the reaction-diffusion-convection evolution equation for the concentration of an autocatalytic species to the incompressible Stokes equations ruling the evolution of the flow velocity in a two-dimensional geometry. The dimensionless parameter of the problem is a solutal Rayleigh number constructed upon the characteristic reaction-diffusion length scale. We show numerically that the asymptotic dynamics is one steady vortex surrounding, deforming, and accelerating the chemical front. This chemohydrodynamic structure propagating at a constant speed is quite different from the one obtained in the case of a pure hydrodynamic flow resulting from the contact between two solutions of different density or from the pure reaction-diffusion planar traveling front. The dynamics is symmetric with regard to the middle of the layer thickness for positive and negative Rayleigh numbers corresponding to products, respectively, lighter or heavier than the reactants. A parametric study shows that the intensity of the flow, the propagation speed, and the deformation of the front are increasing functions of the Rayleigh number and of the layer thickness. In particular, the asymptotic mixing length and reaction-diffusion-convection speed both scale as Ra for Ra>5. The velocity and concentration fields in the asymptotic dynamics are also found to exhibit self-similar properties with Ra. A comparison of the dynamics in the case of a monostable versus bistable kinetics is provided. Good agreement is obtained with experimental data on the speed of iodate-arsenous acid fronts propagating in horizontal capillaries. We furthermore compare the buoyancy-driven dynamics studied here to Marangoni-driven deformation of traveling chemical fronts in solution open to the air in the absence of gravity previously studied in the same geometry [L. Rongy and A. De Wit, J. Chem. Phys. 124, 164705 (2006)].
Photodissociation of alkyl iodides in helium nanodroplets. I. Kinetic energy transfer
Authors: Braun Andreas , Drabbels Marcel
Published: September 2007

The photodissociation of (fluorinated) alkyl iodides in helium nanodroplets at a wavelength of 266nm has been investigated by means of ion imaging techniques. It is found that a significant fraction of the created fragments escapes from the helium droplets. The speed and kinetic energy distributions of these fragments are found to be notably modified with respect to the corresponding gas phase distributions. The fragments, furthermore, show a speed dependent angular distribution. The loss of kinetic energy as well as the reduction of the anisotropy parameter show a strong mass dependence. These observations point to a nonthermal escape process in which the kinetic energy and momentum transfer from the fragments to the solvent is governed by binary collisions with the individual helium atoms making up the droplet. Monte Carlo simulations based on hard-sphere binary collisions substantiate this interpretation of the data.
Photodissociation of alkyl iodides in helium nanodroplets. II. Solvation dynamics
Authors: Braun Andreas , Drabbels Marcel
Published: September 2007

The solvation dynamics of nonthermal species in liquid helium has been investigated by photolyzing alkyl iodide molecules, CH3I, C2H5I, and CF3I, embedded in helium nanodroplets. Iodine and CH3 fragments are found to leave the droplets solvated by a finite number of helium atoms, this in contrast to C2H5 and CF3 fragments. The speed distributions of the IHeN and CH3HeN complexes show a prominent correlation with the degree of solvation N. It is argued that this correlation is caused by a dynamical adjustment of the solvation structure size to the relative speed of the traveling fragments as they pass through the helium bath. The absence of C2H5HeN and CF3HeN complexes is attributed to the large internal energy of these alkyl fragments which leads to a rapid destruction of any possibly formed complexes.
Photodissociation of alkyl iodides in helium nanodroplets. III. Recombination
Authors: Braun Andreas , Drabbels Marcel
Published: September 2007

The recombination of fragments resulting from the photodissociation of (fluorinated) alkyl iodides in helium nanodroplets at a wavelength of 266nm has been investigated by means of ion imaging techniques. It is found that in the case of CH3I an appreciable fraction of the fragments recombine in the aftermath of the photolysis. The proposed mechanism involves a complete translational relaxation of both photofragments inside the nanodroplets followed by geminate recombination of the fragments. In contrast with CH3I, no recombination is observed for CF3I. This is attributed to the larger masses and the different initial kinetic energies of the fragments produced by the photolysis of CF3I, which strongly diminishes the fragment thermalization efficiency.
Photoelectron spectroscopy of hydrated hexafluorobenzene anions
Authors: Eustis Soren N. , Wang Di , Bowen Kit H. , Naresh Patwari G.
Published: September 2007

We present a synergetic experimental/theoretical study of hydrated hexafluorobenzene anions. Experimentally, we measured the anion photoelectron spectra of the anions, C6F6−(H2O)n (n=0–2). The spectra show broad peaks, which shift to successively higher electron binding energies with the addition of each water molecule to the hexafluorobenzene anion. Complementing these results, we also conducted density functional calculations which link adiabatic electron affinities to the optimized geometric structures of the negatively charged species and their neutral counterparts. Neutral hexafluorobenzene-water complexes are not thought to be hydrogen bonded. In the case of C6F6−(H2O)1, however, its water molecule was found to lie in the plane of the hexafluorobenzene anion, bound by two O–H⋯F ionic hydrogen bonds. Whereas in the case of C6F6−(H2O)2, both water molecules also lie in the plane of and are hydrogen bonded to the hexafluorobenzene anion but on opposite ends. This study and that of Schneider et al. [J. Chem. Phys. 127, 114311 (2007), preceding paper] are in agreement regarding the geometry of C6F6−(H2O)1.
Grid-free density functional calculations on periodic systems
Authors: Varga Štefan
Published: September 2007

Density fitting scheme is applied to the exchange part of the Kohn-Sham potential matrix in a grid-free local density approximation for infinite systems with translational periodicity. It is shown that within this approach the computational demands for the exchange part scale in the same way as for the Coulomb part. The efficiency of the scheme is demonstrated on a model infinite polymer chain. For simplicity, the implementation with Dirac-Slater Xα exchange functional is presented only. Several choices of auxiliary basis set expansion coefficients were tested with both Coulomb and overlap metric. Their effectiveness is discussed also in terms of robustness and norm preservation.
Temperature dependence of the colloidal agglomeration inhibition: Computer simulation study
Authors: Barcenas Mariana , Douda Janna , Duda Yurko
Published: September 2007

There exist experimental evidences that the structure and extension of colloidal aggregates in suspensions change dramatically with temperature. This results in an associated change in the suspension rheology. Experimental studies of the inhibitor applications to control the particle clustering have revealed some unexpected tendencies. Namely, the heating of colloidal suspensions has provoked either extension or reduction of the colloidal aggregates. To elucidate the origin of this behavior, we investigate the influence of temperature on the stabilizing effect of the inhibitor, applying an associative two-component fluid model. Our results of the canonical Monte Carlo simulations indicate that the anomalous effect of the temperature may not be necessarily explained by the temperature dependent changes in the inhibitor tail conformation, as has been suggested recently by Won et al. [Langmuir 21, 924 (2005)]. We show that the competition between colloid-colloid and colloid-inhibitor associations, which, in turn, depends on the temperature and the relative concentrations, may be one of the main reasons for the unexpected temperature dependence of inhibitor efficacy.
Experimental and theoretical study of reactivity trends for methanol on Co∕Pt(111) and Ni∕Pt(111) bimetallic surfaces
Authors: Skoplyak Orest , Menning Carl A. , Barteau Mark A. , Chen Jingguang G.
Published: September 2007

Methanol was used as a probe molecule to examine the reforming activity of oxygenates on Ni∕Pt(111) and Co∕Pt(111) bimetallic surfaces, utilizing density functional theory (DFT) modeling, temperature-programed desorption, and high-resolution electron energy loss spectroscopy (HREELS). DFT results revealed a correlation between the methanol and methoxy binding energies and the surface d-band center of various Ni∕Pt(111) and Co∕Pt(111) bimetallic surfaces. Consistent with DFT predictions, increased production of H2 and CO from methanol was observed on a Ni surface monolayer on Pt(111), designated as Ni–Pt–Pt(111), as compared to the subsurface monolayer Pt–Ni–Pt(111) surface. HREELS was used to verify the presence and subsequent decomposition of methoxy intermediates on Ni∕Pt(111) and Co∕Pt(111) bimetallic surfaces. On Ni–Pt–Pt(111) the methoxy species decomposed to a formaldehyde intermediate below 300K; this species reacted at ∼300K to form CO and H2. On Co–Pt–Pt(111), methoxy was stable up to ∼350K and decomposed to form CO and H2. Overall, trends in methanol reactivity on Ni∕Pt(111) bimetallic surfaces were similar to those previously determined for ethanol and ethylene glycol.
Electronic structure of spherical quantum dots using coupled cluster method
Authors: Heidari Ideh , Pal Sourav , Pujari B. S. , Kanhere D. G.
Published: September 2007

2, 6, 12, and 20 electron quantum dots have been studied using coupled cluster at singles and doubles level and extensive multireference coupled cluster (MRCC) method. A Fock-space version of MRCC (FSMRCC) containing single hole-particle excited determinants has been used to calculate low-lying excited states of the above system. The ionization potential and electron affinity are also calculated. The effect of correlation energy on excitation energy and charge density is shown by calculating them at the high density region (low value of density parameter rs) and at the low density region (high value of density parameter rs).
Detonation interaction with an interface
Authors: Lieberman D. H. , Shepherd J. E.
Published: September 2007

Detonation interaction with an interface was investigated, where the interface separated a combustible from an oxidizing or inert mixture. The ethylene-oxygen combustible mixture had a fuel-rich composition to promote secondary combustion with the oxidizer in the turbulent mixing zone (TMZ) that resulted from the interaction. Sharp interfaces were created by using a nitro-cellulose membrane to separate the two mixtures. The membrane was mounted on a wood frame and inserted in the experimental test section at a 45° angle to the bulk flow direction. The membrane was destroyed by the detonation wave. The interaction resulted in a transmitted and reflected wave at a node point similar to regular shock refraction. A detonation refraction analysis was carried out to compare with the measured shock angles. It was observed that the measured angle is consistently lower than the predicted value. An uncertainty analysis revealed possible explanations for this systematic variation pointing to factors such as the incident wave curvature and the role of the nitro-cellulose diaphragm. Analysis of the TMZ and Mach stem formed from the reflection of the transmitted shock wave off the solid boundary were carried out and found to justify the size and strength of these features as a function of the test gas composition. The role of secondary combustion in the TMZ was also investigated and found to have a small influence on the wave structure.
Efficient estimators for quantum instanton evaluation of the kinetic isotope effects: Application to the intramolecular hydrogen transfer in pentadiene
Authors: Vaníček Jiří , Miller William H.
Published: September 2007

The quantum instanton approximation is used to compute kinetic isotope effects for intramolecular hydrogen transfer in cis-1,3-pentadiene. Due to the importance of skeleton motions, this system with 13 atoms is a simple prototype for hydrogen transfer in enzymatic reactions. The calculation is carried out using thermodynamic integration with respect to the mass of the isotopes and a path integral Monte Carlo evaluation of relevant thermodynamic quantities. Efficient “virial” estimators are derived for the logarithmic derivatives of the partition function and the delta-delta correlation functions. These estimators require significantly fewer Monte Carlo samples since their statistical error does not increase with the number of discrete time slices in the path integral. The calculation treats all 39 degrees of freedom quantum mechanically and uses an empirical valence bond potential based on a molecular mechanics force field.
Relativistic Buneman instability in the laser breakout afterburner
Authors: Albright B. J. , Yin L. , Bowers Kevin J. , Hegelich B. M. , Flippo K. A. , Kwan T. J. T. , Fernández J. C.
Published: September 2007

A new laser-driven ion acceleration mechanism has been identified in particle-in-cell simulations of high-contrast-ratio ultraintense lasers with very thin (10s of nm) solid targets [Yin et al., Laser and Particle Beams 24, 291 (2006); Yin et al., Phys. Plasmas 13, 072701 (2007)]. After a brief period of target normal sheath acceleration (TNSA), “enhanced” TNSA follows. In this stage, the laser rapidly heats all the electrons in the target as the target thickness becomes comparable to the skin depth and enhanced acceleration of the ions results. Then, concomitant with the laser penetrating the target, a large accelerating longitudinal electric field is generated that co-moves with the ions. This last phase has been termed the laser “breakout afterburner” (BOA). Earlier work suggested that the BOA was associated with the Buneman instability that efficiently converts energy from the drift of the electrons into the ions. In this Brief Communication, this conjecture is found to be consistent with particle-in-cell simulation data and the analytic dispersion relation for the relativistic Buneman instability.
On nonlinear excitation of voids in dusty plasmas
Authors: Nebbat E. , Annou R. , Bharuthram R.
Published: September 2007

The void, which is a dust-free region inside the dust cloud in the plasma, results from a balance of the electrostatic force and the ion-drag force on a dust particulate that has numerous forms, some of which are based on models whereas others are driven from first principles. To explain the generation of voids, K. Avinash, A. Bhattacharjee, and S. Hu [Phys. Rev. Lett. 90, 075001 (2003)] proposed a time-dependent nonlinear model that describes the void as a result of an instability. We augment this model by incorporating the grain drift and reintroducing the velocity convective term as well as by replacing the modeled ion-drag force by a more accurate one. The analysis is conducted in a spherical configuration. It is revealed that the void formation is a threshold phenomenon, i.e., it depends on the grain size. Furthermore, the void possesses a sharp boundary beyond which the dust density decreases and may present a corrugated aspect. For big size grains, the use of both ion-drag forces leads to voids of the same dimension, though for grains of small sizes, the Avinash force drives voids of a higher dimension. The model shows good agreement with the experiment.
Gaussian basis sets for accurate calculations on molecular systems in gas and condensed phases
Authors: VandeVondele Joost , Hutter Jürg
Published: September 2007

We present a library of Gaussian basis sets that has been specifically optimized to perform accurate molecular calculations based on density functional theory. It targets a wide range of chemical environments, including the gas phase, interfaces, and the condensed phase. These generally contracted basis sets, which include diffuse primitives, are obtained minimizing a linear combination of the total energy and the condition number of the overlap matrix for a set of molecules with respect to the exponents and contraction coefficients of the full basis. Typically, for a given accuracy in the total energy, significantly fewer basis functions are needed in this scheme than in the usual split valence scheme, leading to a speedup for systems where the computational cost is dominated by diagonalization. More importantly, binding energies of hydrogen bonded complexes are of similar quality as the ones obtained with augmented basis sets, i.e., have a small (down to 0.2kcal∕mol) basis set superposition error, and the monomers have dipoles within 0.1D of the basis set limit. However, contrary to typical augmented basis sets, there are no near linear dependencies in the basis, so that the overlap matrix is always well conditioned, also, in the condensed phase. The basis can therefore be used in first principles molecular dynamics simulations and is well suited for linear scaling calculations.
Scanning tunneling microscopy study of metal-free phthalocyanine monolayer structures on graphite
Authors: Nilson Katharina , Åhlund John , Brena Barbara , Göthelid Emmanuelle , Schiessling Joachim , Mårtensson Nils , Puglia Carla
Published: September 2007

Low temperature scanning tunneling microscopy (STM) studies of metal-free phthalocyanine (H2Pc) adsorbed on highly oriented pyrolytic graphite (HOPG) have shown ordered arrangement of molecules for low coverages up to 1 ML. Evaporation of H2Pc onto HOPG and annealing of the sample to 670K result in a densely packed structure of the molecules. Arrangements of submonolayer, monolayer, and monolayer with additional adsorbed molecules have been investigated. The high resolution of our investigations has permitted us to image single molecule orientation. The molecular plane is found to be oriented parallel to the substrate surface and a square adsorption unit cell of the molecules is reported. In addition, depending on the bias voltage, different electronic states of the molecules have been probed. The characterized molecular states are in excellent agreement with density functional theory ground state simulations of a single molecule. Additional molecules adsorbed on the monolayer structures have been observed, and it is found that the second layer molecules adsorb flat and on top of the molecules in the first layer. All STM measurements presented here have been performed at a sample temperature of 70K.
Kinetics of interfacial charges in hybrid GaAs/oligothiophene semiconducting heterojunctions
Authors: Cabanillas-Gonzalez Juan , Gambetta Alessio , Zavelani-Rossi Margherita , Lanzani Guglielmo
Published: September 2007

The authors report on pump-probe measurements in a hybrid heterostructure consisting of a modified oligothiophene layer deposited over a GaAs substrate. Upon photoexcitation of the bilayer with 100fs, a new photoinduced absorption band (PA2) appeared being attributed to oligothiophene cations arising from exciton diffusion and dissociation at the hybrid inorganic/organic interface. An exciton diffusion length in the organic layer of approximately 3.4 nm was estimated.