We show that there can be no direct first order transition between a Fermi liquid and an insulating electronic (Wigner) crystalline phase in a clean two-dimensional electron gas in a metal-oxide-semiconductor field-effect transistor (MOSFET); rather, there must always exist intermediate ``micro-emulsion'' phases, and an accompanying sequence of continuous phase transitions. Among the intermediate phases which we find are a variety of electronic liquid crystalline phases, including stripe-related analogues of classical smectics and nematics. The existence of these phases can be established in the neighborhood of the phase boundaries on the basis of an {\it asymptotically exact} analysis, and reasonable estimates can be made concerning the ranges of electron densities and device geometries in which they exist. They can occur in clean Si MOSFETs in the range of densities in which an ``apparent metal to insulator transition'' has been observed in existing experiments. We...

The Boltzmann equation for transport in semiconductors is projected onto spherical harmonics in such a way that the resultant balance equations for the coefficients of the distribution function times the generalized density of states can be discretized over energy and real space by box integration. This ensures exact current continuity for the discrete equations. Spurious oscillations of the distribution function are successfully suppressed by stabilization based on a maximum entropy dissipation principle avoiding the H-transformation. The derived formulation can be used on arbitrary grids as long as box integration is possible. The new approach works not only with analytical bands but also with full-band structures in the case of holes. Results are presented for holes in bulk silicon based on a full band structure and electrons in a Si NPN BJT. For the first time the convergence of the spherical harmonics expansion is shown for a device and it is found that the quasiballistic transport in nanoscale devices requires an expansion of considerably higher order than the usual first one. The stability of the discretization is demonstrated for a range of grid spacings in the real space and bias points which produce huge gradients in the electron density and electric field. It is shown that the resultant large linear system of equations can be solved memory efficiently by the numerically robust package ILUPACK.

Electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (ESI-MS/ MS) have been used to differentiate the 2-and 4-methylpyridyl isomers of free-base and metallated cationic ␤-vinylpyridylporphyrins. The analysis by ESI-MS/MS of the deuterated analogs and semiempirical calculations of structural and electronic parameters were also undertaken. The two free-base isomers are easily differentiated by ESI-MS/MS but the presence of a metallic center renders differentiation of the metallated isomers less effective. The data acquired show that of all the studied compounds, the free-base 2-methylpyridyl isomer, which was operative in the in vitro photoinactivation of Herpes simples virus, has a different gas-phase behavior. Local distortion of the macrocycle due to the presence of the ␤-vinylpyridyl substituent occurs for all the compounds, but a different electron density distribution can account for the observed gas-phase behavior of this potential virus photoinactivator.

A ballistic calculation of a full quantum mechanical system is presented to study 2D nanoscale devices. The simulation uses the nonequilibrium Green's function (NEGF) approach to calculate the transport properties of the devices. While most available software uses the finite difference discretization technique, our work opts to formulate the NEGF calculation using the finite element method (FEM). In calculating a ballistic device, the FEM gives some advantages. In the FEM, the floating boundary condition for ballistic devices is satisfied naturally. This paper gives a detailed finite element formulation of the NEGF calculation applied to a double-gate MOSFET device with a channel length of 10 nm and a body thickness of 3 nm. The potential, electron density, Fermi functions integrated over the transverse energy, local density of states and the transmission coefficient of the device have been studied. We found that the transmission coefficient is significantly affected by the top of the barrier between the source and the channel, which in turn depends on the gate control. This supports the claim that ballistic devices can be modelled by the transport properties at the top of the barrier. Hence, the full quantum mechanical calculation presented here confirms the theory of ballistic transport in nanoscale devices.

The theoretical electronic densities of states for both the valence and conduction bands are presented for the tetrahedral semiconductors CdTe, HgTe and their alloy Hg 0:5 Cd0:5Te based on band structures computed using the empirical pseudopotential method. For the ternary alloy HgCdTe, we have coupled the virtual crystal approximation with the pseudopotential method. Various quantities such as the energy levels, the refractive index and the transverse e ective charge are calculated. The spin-orbit splitting is included in our calculations.

A set of heavily doped Al0.48In0.52As samples grown by molecular beam epitaxy on InP (Fe) substrates was investigated using the photoreflectance (PR) technique. The spectra at 300 K are characterized by a transition in the vicinity of the InP energy gap, followed by strongly damped Franz-Keldysh oscillations (FKOs) which do not appear when the spectra are obtained at 77 K. The builtin electric field estimated from FKOs shows a small doping dependence but is substantially affected by the inclusion of a thin layer of AlxGayIn1-x-yAs (x≡0.22) at the interface between InP (Fe) and AlInAs:Si. In order to explain these results, a model based on the discontinuity of the energy bands in the InP/AlInAs and InP/AlGaInAs/AlInAs systems and also on the matching of the Fermi levels between the different materials is suggested.

The charge-density distribution in 2H-chromene-2-thione (2-thiocoumarin), C 9 H 6 OS, has been determined from X-ray diffraction data measured at 90 K using a CCD detector, to a resolution of sin /! < 1.08 A Ê À1 . A multipolar-atom density model was ®tted against 6908 re¯ections with I > 2'(I ) [R(F ) = 0.021, wR(F) = 0.022, goodness of ®t = 1.81] in order to generate the difference Fourier maps. The topological properties of the molecular electron density in terms of the bond critical points and the evaluation of the dipole moment show that the molecular dipole moment in the crystal is higher than the corresponding value derived from theoretical calculations.

The GPS radio occultation technique is a rather simple and inexpensive tool for profiling the electron density of the entire ionosphere from satellite orbit heights down to the bottomside. No other profiling technique (bottomside/topside vertical sounding, incoherent scatter) unifies profiling through the entire ionosphere with global coverage. First results of ionospheric radio occultation (IRO) measurements carried out on board the German CHAMP satellite are reported. To inform potential users we review IRO data products operationally generated in DLR/IKN Neustrelitz and distributed via the Information System and Data Center (ISDC) of GFZ. Methods and algorithms applied for product generation including their limitations in accuracy and spatial resolution are addressed. We present the status and results of product validation with independent data sources. The achieved accuracy of the retrieved electron density profiles is estimated in particular by comparing the IRO results with independent data obtained from vertical sounding stations on Earth and from the Langmuir probe on board CHAMP. It is concluded that CHAMP-IRO measurements have the potential to establish global data sets of electron density profiles for developing and improving global ionospheric models and to provide operational space weather information.

Recent theoretical model simulations of the ionospheric response to geomagnetic storms have provided the understanding for the development of an empirical storm-time ionospheric model (STORM). The empirical model is driven by the previous time-history of a p , and is designed to scale the quiet-time F-layer critical frequency (f o F 2 ) to account for storm-time changes in the ionosphere. The model provides a useful, yet simple tool for modeling of the perturbed ionosphere. The quality of the model prediction has been evaluated by comparing with the observed ionospheric response during the Bastille Day (July 2000) storm. With a maximum negative D st of −290 nT and an a p of 400, this magnetic perturbation was the strongest of year 2000. For these conditions, the model output was compared with the actual ionospheric response from all available stations, providing a reasonable latitudinal and longitudinal coverage. The comparisons show that the model captures the decreases in electron density particularly well in the northern summer hemisphere. In winter, the observed ionospheric response was more variable, showing a less consistent response, imposing a more severe challenge to the empirical model. The value of the model has been quantified by comparing the root mean square error (RMSE) of the STORM predictions with the monthly mean. The results of this study illustrate that the STORM model reduces the RMSE at the peak of the disturbance from 0.36 to 0.22, a significant improvement over climatology. Physics 204: 317-324, 2001.

The topside ionosphere modeling suffers from the relative scarcity of experimental electron density data. Different modeling approaches have been applied in empirical models, like IRI, and profilers, like NeQuick. Comparing ISIS-2 topside electron density profiles with IRI and NeQuick models weak points in their topside formulation are identified and analysed. The IRI topside consists in two constant gradient sections, with a transition height kept at a fixed height in its domain. Tests of transition height and gradient modifications are presented, based on experimental evidences and model efforts. NeQuick model has a topside formulation based on a semi-Epstein layer, governed by an empirical shape parameter k. An alternative formulation is presented based on k values derived from experimental profiles.

An algorithm is presented for carrying out decomposition of electronic charge density into atomic contributions. As suggested by Bader [R. Bader, Atoms in Molecules: A Quantum Theory, Oxford University Press, New York, 1990], space is divided up into atomic regions where the dividing surfaces are at a minimum in the charge density, i.e. the gradient of the charge density is zero along the surface normal. Instead of explicitly finding and representing the dividing surfaces, which is a challenging task, our algorithm assigns each point on a regular (x, y, z) grid to one of the regions by following a steepest ascent path on the grid. The computational work required to analyze a given charge density grid is approximately 50 arithmetic operations per grid point. The work scales linearly with the number of grid points and is essentially independent of the number of atoms in the system. The algorithm is robust and insensitive to the topology of molecular bonding. In addition to two test problems involving a water molecule and NaCl crystal, the algorithm has been used to estimate the electrical activity of a cluster of boron atoms in a silicon crystal. The highly stable three-atom boron cluster, B 3 I is found to have a charge of À1.5 e, which suggests approximately 50% reduction in electrical activity as compared with three substitutional boron atoms.

Prediction of LF signal strengths is usually done by using a single "average" model of the ionosphere. However, this approach suffers from the fact that signal strength is a nonlinear function of ionospheric parameters. Hence the "average" ionosphere does not always give the correct average signal strength. In addition, the standard deviation of signal strength varies considerably with position, in general being larger in the vicinity of modal interference nulls. A model where the daytime ionosphere is treated statistically is presented. The propagation model used is an earth ionosphere waveguide. The ionospheric electron density profile is specified by a reference height h' and slope fl. These parameters are assumed to be distributed in a jointly normal distribution. Best fit values for this model are obtained using measurements made at two points on either side of a deep modal interference null located about 1000 km from a 59-kHz transmitter. The measured signal strength mean, and standard deviation were used to obtain a best fit mean, standard deviation and correlation coefficient for the fi and h' distribution. The resulting ionospheric parameters give signal statistics that fit the measured mean and standard deviation at both monitoring points within 1 dB.

For the radiofluorination of benzenes and benzene derivatives, the electrophilic reaction with [ 18 F]F 2 is a very common route. Yet, aromatic nucleophilic substitution (S N Ar) by n.c.a [ 18 F]fluoride, which can be produced efficiently in high amounts, has been considered to be very desirable. However, to facilitate 18 F-labelling via S N Ar at an electron rich aromatic system, an appropriate leaving group must be present together with an auxiliary group in ortho or para position to the leaving group. An interesting alternative for the auxiliary group is the heteroatom of a heteroaromatic system, for which pyridine is a leading example. Dolci et al. (J Label Compd Radiopharm 42:975-985, 1999) have evaluated the scope of the nucleophilic aromatic fluorination of 2-substituted pyridine rings using the activated K [ 18 F]F-K 222 complex. As methyl and methoxy groups are known to enhance the electron density of an aromatic system by the ?I and the ?M effect, respectively, S N Ar is unlikely to occur. Until now, the effect of these substituents has not been studied towards the 18 F-radiofluorination of substituted 2-nitropyridines by use of [ 18 F]fluoride. Therefore, we have investigated the effect of methoxy and methyl groups in 2-nitropyridines. The results showed that 3-methoxy-2nitropyridine and 3-methyl-2-nitropyridine can efficiently be substituted by [ 18 F]fluoride with high RCY's (70-89%) in short reaction times (1-30 min) at a reaction temperature of 140°C. Moreover, 3-methoxy-6-methyl-2-[ 18 F]fluoropyridine was obtained from the corresponding nitro-precursor in a high yield of 81 ± 1% after 30 min at 140°C. In case of 2-nitropyridines data indicates the effect of methyl and methoxy groups on S N Ar to be of minor importance.

Spin-polarized photoemission was employed to investigate the temperature dependence of the exchange splitting of Gd(0001) for both the surface state near the Fermi level Fr and the 5d bulk bands. The bulk bands at 1-2 eV below EF show Stoner-like behavior where two peaks with opposite spin polarization shift toward each other as the temperature increases. In contrast, the temperature dependence of the surface state indicates spin-mixing behavior due to fluctuating local moments. These differences are attributed to the degree of itinerancy of the 5d electrons in the two cases.

To gain a better understanding of the factors involved in textural changes in Mozzarella cheese during storage, electron microscopic imaging was used to compare the ultrastructure of the protein matrix on the day of stretching (0 weeks) and after 6 weeks of storage at 4°C. Low-fat and full-fat cheeses were prepared from nonhomogenized milk and from milk homogenized at 10.3 and 17.2 MPa. Analysis of the stained electron density distribution in image transforms of the protein matrix revealed that spacing patterns of regular structures with the dimensions of casein submicelles changed during storage, in most cases forming larger aggregates at longer average spacings, but there were no significant changes in total electron density between fresh and stored cheeses. These effects were related to proteolysis. Submicelles in low-fat cheeses homogenized at 17.2 MPa reorganized differently, a likely result of decreased proteolysis. Submicelles formed part of a new membrane around fat globules in cheeses made from homogenized milk, as previously observed. Changes in submicelle size and distribution could explain some of the differences in texture properties between fresh and stored Mozzarella cheeses, and between products made from homogenized and nonhomogenized milk.

We review the information derived from whistler spectrograms (recorded on the ground) about the equatorial magnetic field, equatorial electron density, total electron content of a flux tube, downward transport of flux of electrons, large scale electric fields in the equatorial region, characteristic properties of the ducts present in the plasma, and electron temperature. The above parameters derived from the analysis of whistlers recorded at low latitude ground stations are also included. Thus, it is demonstrated that the probing of the entire plasmasphere can be easily achieved by recording whistler waves at ground based stations scattered in latitude and longitude. ,c>

The interaction of a CO molecule with the Fe(100) surface is analyzed by the topology of the Laplacian of the electronic charge density −V2r. This analysis shows that the atomic graph of the top atoms of the Fe(100) surface is a cube that exposes a face, F top , with a ''hole'' of charge or (3,+1) critical point of −V2r, whereas the corresponding graph for the second layer atoms is an octahedron that exhibits a vertex, V sec , protruding above the Fe(100) surface, with a ''peak'' of charge or (3,−3) critical point of −V2r. Atomic graphs determined for the CO molecule show that the carbon atom has a non-bonded vertex, C Vnb , and a torus, C torus , of charge depletion perpendicular to the C-O bond direction. As the CO molecule approaches the surface, electron transfer towards the tilted CO in turn induces two non-bonded critical points, O Vnb , on the O atom. In accordance with experimental and theoretical results, the topological theory of the Laplacian suggests that the preferred pathway for CO dissociation corresponds to the tilted orientation on the surface, in which the attractive interaction C torus -V sec is enhanced by the interaction of the two O Vnb with the F top on two Fe top atoms of the (100) surface.

ENIGMA (Electron and Nuclear Image Generator by Max-ent Analysis) is a program package to evaluate three-dimensional electron and nuclear density from X-ray and neutron diffraction data by using the maximum-entropy method (MEM). Compared with the previous program package MEED, ENIGMA saves computing time and frees memory space at the same time by employing parallel data processing. The fast Fourier transformation (FFT) technique is also implemented. As a consequence of these improvements, the MEM analysis by ENIGMA becomes applicable to huge systems, such as proteins and polymers, when the phased structure factors are provided. The package is transferable to a wide variety of parallel computers, because it is written in Fortran 90 and a standard message-passing interface (MPI).

A lag-profile data collection mode has recently become operational at Millstone Hill. A new data analysis technique has been developed in which one or more matrices of ion-line lagged products are analyzed simultaneously, along with any available a priori information on ionospheric and system parameters such as ionosonde and plasma-line estimates offoF2. The analysis yields spline function estimates of height-varying ionospheric parameter profiles. The effect of the full two-dimensional (range-lag) radar ambiguity function is included in this analysis. Millstone Hill single-pulse lag profile data are presented and analyzed and the new technique is shown to produce ionospheric parameter profile estimates with significantly better height resolution than would be possible if the lag profile matrix were first divided into ACFs by application of a summation rule. 1.

Theoretical results are presented regarding the incorporation of Scandium into wurtzite GaN and InN binaries. The electric, optical and piezoelectric properties of the resulting ScGaN and ScInN systems are reported by using first-principles Local-density approximation (LDA) within density functional theory (DFT), Berry phase approach within modern theory of polarization and phonon calculations within the density functional perturbation theory. Our results predict the existence of breaking-symmetry structural phase transition in ordered Sc0.5Ga0.5N and Sc0.5In0.5N alloys when subjected to a compressive or tensile strain. Moreover, our results demonstrate the existence of symmetry preserving pressure-induced isostructural phase transitions in ordered ScGaN and ScInN systems for different Sc concentrations. It has been shown that the existence of isostructural phase transitions leads to dramatic changes in optical, acoustic, and piezoelectric properties of ordered ScGaN and ScInN systems under high pressure. In particular, this study demonstrates that the existence of first-order isostructural phase transitions in Sc1Ga1N2 at a critical hydrostatic pressure of 12.3 GPa leads to a huge enhancement of piezoelectricity (i.e., the e33 piezoelectric coefficient adopts a huge value as large as 13 C/m 2). In addition, It has been shown that ordered Sc0.5Ga0.5N and Sc0.5In0.5N alloys exhibit tremendous piezoelectric response, associated with a breakingsymmetry phase transition from nonpolar P 63/mcc(D 6h) space group to a polar P 63mc(C6v) structure, at fixed Ga, In and Sc compositions, as a function of the in-plane compressive and tensile strains. We also reveal the reason behind, and consequences of, these unusual properties associated with the strain-induced and pressure-induced structural phase transitions in the novel ScGaN and ScInN ordered structures.

We report some new results on the Anomalous Hall effect induced by the Berry phase in momentum space. Our main calculations are performed within the model of a two-dimensional electron gas with the spin-orbit interaction of Rashba type, taking into account the scattering from impurities. We demonstrate that such an "intrinsic" mechanism can really dominate but there is a competition between the geometric Berry-phaseinduced term σ II xy in the Hall conductivity and the impurity-induced term σ I xy , related to the contribution of the states in the vicinity of the Fermi surface. We also show that the contribution to the Hall conductivity from the electron states close to the Fermi surface has the intrinsic property as well, and it does not vanish in the clean limit. The main effect of the impurity-related contribution is a possible change of sign of the off-diagonal conductivity. The resulting magnitude and sign of the Hall conductivity strongly depend on the electron density in the system.

Stages in the development of polyphosphate bodies in the blue-green alga, Plectonema boryanum, grown under continuous illumination in the presence of excess phosphate, are reported. During the first stage, an electronlucent area appears near the nucleoplasm or cross walls; it gradually increases to a size approximately equal to that of the final polyphosphate body. In this area a porous structure of medium electron density develops, while simultaneously electron-dense material, interpreted as polyphosphate, is deposited in the adjacent cytoplasm. Eventually, this material seems to penetrate into the porous structure. When the polyphosphate bodies are fully formed, the surrounding cytoplasm does not contain detectable amounts of polyphosphate. The formation of polyphosphate bodies in P. boryanum is compared with that in some bacterial species.

We present a fully Hamiltonian and computationally efficient scheme to include the electrostatic effects due to the classical environment in a Car-Parrinello mixed quantum Mechanics/molecular mechanics ͑QM/MM͒ method. The polarization due to the MM atoms close to the quantum system is described by a Coulombic potential modified at short range. We show that the functional form of this potential has to be chosen carefully in order to obtain the correct interaction properties and to prevent an unphysical escape of the electronic density to the MM atoms ͑the so-called spill-out effect͒. The interaction between the QM system and the more distant MM atoms is modeled by a Hamiltonian term explicitly coupling the multipole moments of the quantum charge distribution with the classical point charges. Our approach remedies some of the well known deficiencies of current electrostatic coupling schemes in QM/MM methods, allowing molecular dynamics simulations of mixed systems within a fully consistent and energy conserving approach.

The effect of topological disorder on the cohesive energy and the electronic density of states of amorphous Si and Ge is investigated. The methods used include moment expansions and various exactly soluble models that include the Bethe lattice and Husimi cactus lattices, for which an extremely compact derivation of the density of states is presented. An isolated topological defect in a diamond cubic lattice is also studied. The question of the existence of square-root band edges in topologically disordered systems is examined. A review of recent experimental evidence, relating to the shape of the density of states, is given.

We extend previous work on quantitative structure-activity and structure-property relationships using molecular descriptors based on quantities determined from the momentum-space (p-space) electron density. In particular, we introduce new molecular descriptors that are related to the p-space information entropy. We also examine the use of a simple molecular shape descriptor, X. For the LC 50 toxicity data of a series of saturated alcohols, the simple shape descriptor is found to be remarkably successful, and a correlation is observed between X and the entropy-like p-space descriptors. We develop a promising 13-descriptor regression model for the log P values of a set of 76 chemically diverse molecules. Similar approaches, combining simple classical parameters with p-space quantities, are likely to prove useful for more complex QSAR/QSPR problems. q

We report a detailed study of the electronic and structural properties of the 39K superconductor \mgbtwo and of several related systems of the same family, namely \mgalbtwo, \bebtwo, \casitwo and \cabesi. Our calculations, which include zone-center phonon frequencies and transport properties, are performed within the local density approximation to the density functional theory, using the full-potential linearized augmented plane wave

Automatic scaling of bottomside Digisonde ionograms gives the E and F region echo traces with high accuracy. Polarization and incidence angle information in the ionogram enables the extraction of the vertical, ordinary polarization echo trace from quiet as well as disturbed ionograms. The scaling algorithm is tested with Digisonde ionograms from Goose Bay, Labrador, which show spread F about 50% of the time. Despite these disturbed conditions, fo F 2 is determined within « MHz for 475 ionograms out of 577 during January 1980. A profile inversion algorithm calculates the electron density profile from the autoscaled h'(f) points. Parabolic profile shapes are assumed for the E region and the valley between the E and the F layer. The F layer is approximated by a single sum of Chebyshev polynomials, and the entire profile is described by a set of 16 numerical values. The CPU time required for ionogram scaling and electron density profile calculation is 14 s on a Cyber 71 computer. In addition to the profile the program outputs all relevant ionogram parameters: critical frequencies, minimum virtual heights, frequency and range spread, average echo amplitudes, and Doppler shifts. 1. INTRODUCTION It has long been recognized that a network of bottomside ionosondes provides a wealth of ionospheric information valuable for research and communication [-Davies, 1966]. The tedious effort involved in scaling and evaluating the conventional ionograms has sometimes discouraged the non-ionosondespecialist from using the ionogram data. Automatic scaling of vertical ionograms is therefore a desirable goal. Only nowadays when advanced digital sounders like the Digisonde [Bibl and Reinisch, 1978] are available can this goal be reached. What we want to show in this paper is an autoscaling algorithm that works with both quiet and disturbed ionograms, and that vertical electron density profiles can routinely be determined at each Digisonde station that measures polarization and incidence angles. Ionograms from the Goose Bay Ionospheric Observatory (64.6øN geomagnetic) of the Air Force Geophysics Laboratory form the data base of the study. These ionograms display a large variety of features: quiet and disturbed daytime recordings, spread F during the night, the mid-latitude trough moving over the station, fast variation of the ionospheric parameters, and frequent absorption events. The typical features of a quiet daytime ionogram are shown in Figure l:an E trace, transition to the F trace, splitting in O and X polarization echoes, and two-hop echoes. The lower part of the figure shows the amplitude ionogram, the upper part the so-called status ionogram, categorizing the echo type for each amplitude pixel. The ionogram distinguishes O and X echoes and vertical and off-vertical echoes as well as positive and negative Doppler shifts. The scaling algorithm extracts the vertical ordinary echo trace from which the electron density profile is calculated. Section 2 describes the algorithm for the autoscaling of the bottomside Digisonde ionograms, and section 3 explains the profile inversion technique for autoscaled data. Reference is frequently made to the two earlier papers in this sequence of three [Reinisch and Huang Xueqin, 1982; Huang Xueqin and Reinisch, 1982], which are cited as part 1 and part 2.

A novel method for determination of mass attenuation coefficient of x-rays employing NaI (Tl) detector system and radioactive sources is described.in this paper. A rigid geometry arrangement and gating of the spectrometer at FWHM position and selection of absorber foils are all done following detailed investigation, to minimize the effect of small angle scattering and multiple scattering on the mass attenuation coefficient, µ/ρ, value. Firstly, for standardization purposes the mass attenuation coefficients of elemental foils such as Aluminum, Copper, Molybdenum, Tantalum and Lead are measured and then, this method is utilized for dosimetric interested material (sulfates). The experimental mass attenuation coefficient values are compared with the theoretical values to find good agreement between the theory and experiment within one to two per cent. The effective atomic numbers of the biological substitute material are calculated by sum rule and from the graph. The electron density of dosimetric material is calculated using the effective atomic number. The study has discussed in detail the attenuation coefficient, effective atomic number and electron density of dosimetric material/biological substitutes.

We report initial measurements of ionospheric propagation parameters, particularly the total electron content (TEC), using the recently launched FORTE satellite. FORTE, which orbits the Earth at an altitude of 800 km and an inclination of 70°, contains a set of wideband radio receivers whose output is digitally recorded. A specialized triggering circuit identifies transient, broadband radio events, which include radiation from lightning, transionospheric pulse pairs, and man-made sources. Event data are transmitted to the ground station for analysis. In this paper we examine signals transmitted from an electromagnetic pulse generator operated at Los Alamos. The transmitter produces nearly impulsive signals in the VHF range. The received signal is dispersed by the ionosphere, and the received signal can be analyzed to deduce the total electron content along the path. By comparing the slant TEC thus measured with results from a ray-tracing code, we can deduce the vertical TEC to 800 km. Data from eight passes are presented. These types of data (in larger quantities) are of interest to operators of radar altimeters, who need data to corroborate their corrections for the ionospheric TEC. The combination of FORTE TEC data to 800 km and TEC measurements to 20,000 km (the Global Positioning System orbital altitude) can provide useful information for assessing the validity of models of plasmaspheric electron density. Initial estimates of the plasmaspheric density, on two daytime passes, are about 6 TECU. The signal received by FORTE, which is linearly polarized at the transmitter, is split into two magnetoionic modes by the ionosphere. The receiving antenna is also linearly polarized and therefore receives both modes. By measuring the beat frequency between the two modes, we can deduce the product of the geomagnetic field and the cosine of the angle between the field and the propagation vector. The possibility of using the measured slant TEC and the beat frequency to geolocate impulsive signals is discussed.

mation is accompanied by a large increase in electrical conductivity; the low-temperature phase is a protonic con-Potassium hydrogen fluoride (KHF 2 ) crystallizes in the tetragonal system, space group I4/mcm, Z ‫؍‬ 4, a ‫؍‬ 5.668(2) Å , ductor, whereas Kϩ, F-, and [FHF]-groups are all very c ‫؍‬ 6.801(7) Å , under ambient conditions. Energy-dispersive mobile in the cubic phase (3-5). A high-pressure conduc-X-ray diffraction was used to study the compression behavior tivity study by Bradley et al. (6) traced the tetragonal-cubic of KHF 2 in a diamond anvil cell. Unit cell parameters were transition up to 5.7 GPa, where it occurred at 245ЊC. The determined at ten different static pressures between 1 atm and near-isothermal slope in P-T space for this transition is a 12.3 GPa, and the calculated cell volumes were found to fit consequence of the large entropy increase on formation a third-order Birch-Murnaghan equation of state with K 0 ‫؍‬ of the cubic phase (23.8 J/mol K; 7, 8) despite the large 25.1(15) GPa and K ‫؍‬ 10.0(19). The compressibility is greatest volume increase (ϩ12.7% at 1 bar). There is a possibility by a factor of two parallel to z despite the short K-K distances of ordered pyrite-like KHF 2 being stable at high pressure in this direction, implying that F-F repulsion controls the comand lower temperatures than the disordered structure or pressibility behavior. A reconnaissance study to much higher the tetragonal phase. Orientational ordering causes a 13% pressure showed that KHF 2 remains in this structure down to volume decrease in NaSH (9). Such a volume change com-67.6% of its ambient volume, at a pressure estimated as 50 bined with loss of configurational entropy might be suffi-

Concerns over the health effects of nanomaterials in the environment have created a need for microscopy methods capable of examining the biological interactions of nanoparticles (NP). Unfortunately, NP are beyond the diffraction limit of resolution for conventional light microscopy (~200 nm). Fluorescence and electron microscopy techniques commonly used to examine NP interactions with biological substrates have drawbacks that limit their usefulness in toxicological investigation of NP. EM is labor intensive and slow, while fluorescence carries the risk of photobleaching the sample and has size resolution limits. In addition, many relevant particles lack intrinsic fluorescence and therefore can not be detected in this manner. To surmount these limitations, we evaluated the potential of a novel combination of darkfield and confocal laser scanning microscopy (DF-CLSM) for the efficient 3D detection of NP in human lung cells. The DF-CLSM approach utilizes the contrast enhancements of da...

The Naval Research Laboratory (NRL) three-dimensional simulation code SAMI3/ESF is used to study the long time evolution of equatorial spread F (ESF) bubbles. The ESF bubbles are modeled until they stop rising and become "fossils," with results analyzed to address previously-untested hypotheses. Specifically, it has been suggested that bubbles stop rising when either the local electron density inside the bubble is equal to that of the nearby background or the fluxtube-integrated electron density inside the bubble is equal to that of the nearby background. It is shown that equatorial bubbles stop rising when the magnetic flux-tube-integrated ion mass density inside the bubble equals that of the surrounding background ionosphere. In the case of a singleion ionosphere this reduces to the condition that the fluxtube-integrated electron densities are in balance, consistent with the hypothesis of Mendillo et al. (2005). Citation: Krall,

Single and double pulse laser-induced breakdown spectroscopy (LIBS) was carried out on aluminum samples in air. In the case of double pulse excitation, experiments were conducted by using the same laser source operated at the same wavelength (1064 nm in most cases here presented). A lowering of the second pulse plasma threshold was observed, together with an overall enhancement in line emission for the investigated time delay between the two pulses (40-60 ms). The laser-induced plasma originated by a single and double pulse was investigated near ignition threshold with the aim to study possible dynamical mechanisms in different regimes. Currently available spectroscopic diagnostics of plasma, such as the line broadening and shift due Stark effects, have been used in the characterization in order to retrieve electron densities, while standard temperature measurements were based on Boltzmann plot. Plasma relevant parameters, such as temperature and electron density, have been measured in the plasma decay on a long time scale, and compared with crater shape (diameter and inferred volume). The comparison of double with single pulse laser excitation was carried out while keeping constant the energy per pulse; the influence of laser energy was investigated as well. Results here obtained suggest that use of the double pulse technique could significantly improve the analytical capabilities of LIBS technique in routine laboratory experiments. ᮊ

The complex dielectric function ͑DF͒ of wurtzite InN and GaN as well as zinc blende GaN was measured by spectroscopic ellipsometry between 14 and 32 eV with synchrotron radiation. In this spectral region, the DF of InN and GaN originates from Ga 3d and In 4d core level transitions to unoccupied conduction-band states. The Ga 3d and In 4d electronic states are highly localized and show almost no dispersion. We use these core states as a reference in order to probe the conduction bands. For this purpose, the imaginary part is compared to the density of empty p-orbital-like electronic states located around the cation atomic site, as calculated by densityfunctional theory in the local density approximation. The constant splitting of absorption features in the DF is attributed to the spin-orbit splitting of the d states. ⌬d 5/2−3/2 is found to be 0.82 eV for the In 4d and 0.41 eV for the Ga 3d level, respectively. On wurtzite samples with the c axis in the surface plane, ellipsometry measurements give access to both independent dielectric tensor components ʈ and Ќ , respectively. The observed anisotropy is induced by a directional dependence of empty p states yielding a p ʈ -DOS ͑density states͒ different to the p Ќ -DOS.

The ultraviolet (UV) spectra of Mira variable stars have been studied for nearly three decades. The International Ultraviolet Explorer (IUE) space telescope observed these stars at both low and high dispersion. Later the Hubble Space Telescope (HST) obtained high-dispersion spectra both with the High Resolution Spectrograph (HRS) and the Space Telescope Imaging Spectrograph (STIS). This paper displays a STIS spectrum of the cool Mira-type variable star R Leo taken on 31 December 1998. On this date R Leo was at phase 0.37 in its light-curve cycle. This spectrum shows a large number of emission lines and identifications are made for nearly 200 of these features. Many of these emission lines were previously unrecorded in IUE and HRS spectra of Miras, such as the Fe II (UV33, 35, 158, 160, 161, 180, 181), Mn II (UV38), V II (UV43, 73), Zr II (UV58), and the Ni II (UV36) multiplets. The electron density diagnostic multiplet of C II] (UV0.01) gives an electron density of 10 9 cm −3 for R Leo at this phase. This is similar to the electron density found for the Mira star R Hya at phase 0.26 obtained with a HST/HRS spectrum. Finally, the photospheric spectrum was detected from 2980Å (the long wavelength cutoff) down to 2450Å.

We present high resolution spectrophotometric data for a sample of 34 planetary nebulae with [WC] spectral type central stars (WRPNe) in our Galaxy. The observed objects cover a wide range in stellar characteristics: early and late [WC] type stars, as well as weak-emission line stars (WELS). Physical conditions in the nebulae (electron density and temperatures) have been obtained from various diagnostic line ratios, and chemical abundances have been derived with the usual empirical scheme. Expansion velocities were estimated in a consistent manner from the line profiles for most objects of the sample. A statistical study was developed for the derived data in order to find fundamental relationships casting some light on the evolutionary status of WRPNe. We found evidence for a strong electron temperature gradient in WRPNe which is related to nebular excitation. Such a gradient is not predicted in simple photoionization models. Abundance ratios indicate that there seems to be no preferential stellar mass for the Wolf-Rayet phenomenon to occur in the nucleus of a planetary nebula. Two objects, M 1-25 and M 1-32, were found to have a very small Ne/O ratio, a property difficult to understand. We reexamined the relation between the nebular properties of the WRPNe and the spectral types of the central stars. Our data confirm the trend found by other authors of the electron density decreasing with decreasing spectral type, which was interpreted as evidence that [WC] stars evolve from late to early [WC] types. On the other hand, our data on the expansion velocities do not show the increase of expansion velocity with decreasing spectral type, that one might expect in such a scenario. Two objects with very late [WC] type central stars, K 2-16 and PM 1-188, do not follow the general density sequence, being of very low density for their spectral types. We suggest that the stars either underwent a late helium flash (the "born again" scenario) or that they have had a particularly slow evolution from the AGB. The 6 WELS of our sample follow the same density vs.

The DNA-containing capsid of canine parvovirus (CPV) is analyzed Department of Chemistry & Institute of Molecular following atomic refinement at 2.9 Å resolution. The capsid contains 60 Biophysics, Florida State copies of the capsid protein related by icosahedral symmetry. The atomic University Tallahassee model has been extended from the first residue (Gly37) of the unrefined 3.25 Å structure towards the N terminus. The electron density shows that FL 32306-3015, USA 087% of the capsid proteins have N termini on the inside of the capsid, but for 013%, the polypeptide starts on the outside and runs through one of the pores surrounding each 5-fold axis, explaining apparently conflicting antigenic data. Analysis of potential hydrogen bonds reveals 050% more secondary structure than previously apparent. Most of the additional secondary structure are in the 71 and 221 residue-long loop insertions between b-strands E and F and G and H, forming subunitbridging sheets that likely add specificity to assembly interactions. Structural analysis of the extensive subunit interactions around the 3-fold axes shows that assembly is a multistep process with loops intertwining following initial contact. Estimated free energies of association suggest that the formation of 3 and 5-fold contacts likely takes precedence over 2-fold interactions. Energies for initial association into trimers or pentamers would be similar, but the intertwining of loops about the 3-fold axis adds an additional large activation barrier to dissociation. Analysis of the surfaces of the assembled capsid shows a surprising lack of basic amino acids that might have been expected to interact with the negatively charged phosphoribose backbone of the DNA. Instead, uncharged polar and van der Waal's interactions predominate in the packaging of single-stranded DNA into the capsid.