Spectral Energies Research Articles

Temperature dependence of 2DEG and excitonic optical transitions in AlGaN/GaN heterostructures on SiC

Four (4) unique optical transitions are reported in both the emission and reflection spectra of high-quality AlGaN/GaN heterostructures. Study of the shifts of spectral peak energies and their intensity variations with temperature, reveal that these transitions arise from Free Exciton recombination and transitions between the A- and B-valence bands and the excited states of the 2-dimensional electron gas (2DEG) at the heterointerface. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Turbulent flow over a dune: Green River, Colorado

AbstractDetailed echo‐sounder and acoustic Doppler velocimeter measurements are used to assess the temporal and spatial structure of turbulent flow over a mobile dune in a wide, low‐gradient, alluvial reach of the Green River. Based on the geometric position of the sensor over the bedforms, measurements were taken in the wake, in transitional flow at the bedform crest, and in the internal boundary layer. Spatial distributions of Reynolds shear stress, turbulent kinetic energy, turbulence intensity, and correlation coefficient are qualitatively consistent with those over fixed, two‐dimensional bedforms in laboratory flows. Spectral and cospectral analysis demonstrates that energy levels in the lee of the crest (i.e. wake) are two to four times greater than over the crest itself, with minima over the stoss slope (within the developing internal boundary layer). The frequency structure in the wake is sharply defined with single, dominant peaks. Peak and total spectral and cross‐spectral energies vary over the bedform in a manner consistent with wave‐like perturbations that ‘break’ or ‘roll up’ into vortices that amalgamate, grow in size, and eventually diffuse as they are advected downstream. Fluid oscillations in the lee of the dune demonstrate Strouhal similarity between laboratory and field environments, and correspondence between the peak frequencies of these oscillations and the periodicity of surface boils was observed in the field. Copyright © 2005 John Wiley & Sons, Ltd.

Reliable results for the Isotropic Dipole – Dipole and Triple – Dipole Dispersion Energy Coefficients for Interactions involving Formaldehyde, Acetaldehyde, Acetone, and Mono - , Di - , and Tri - Methylamine

Pseudo - spectral dipole oscillator strengths and excitation energies, which are discrete representations of previously developed recommended continuous dipole oscillator strength distributions(DOSDs), are presented for the ground state formaldehyde, acetaldehyde, acetone, and mono - , di - , and tri - methylamine molecules. These pseudo - DOSDs, together with previously published pseudo - DOSDs for other atoms and molecules, are used to evaluate the dipole - dipole and the triple - dipole dispersion energy coefficients for all the two - body and three - body interactions between H$_2$CO, CH$_3$CHO, (CH$_3$)$_2$CO, CH$_3$NH$_2$, (CH$_3$)$_2$ NH, and (CH$_3$)$_3$N, and between these molecules and forty - four other species, namely Cl$_2$, SiH$_4$, SiF$_4$, CCl$_4$, H, Li, He, Ne, Ar, Kr, Xe, SF6, HF, HCl, HBr, SO$_2$, CS$_2$, OCS, H$_2$, N$_2$, O$_2$, NO, N$_2$O, H$_2$O, H$_2$S, NH$_3$, CO, CO$_2$, the normal alkanes CH$_4$, C$_2$H$_6$, C$_3$H$_8$, C$_4$H$_{10}$, C$_5$H$_{12}$, C$_6$H$_{14}$, C$_7$H$_{16}$ and C$_8$H$_{18}$, the 1-alkenes C$_2$H$_4$, C$_3$H$_6$ and C$_4$H$_8$, C$_2$H$_2$, C$_6$H$_6$, and the primary alcohols CH$_3$OH, C$_2$H$_5$OH and C$_3$H$_7$OH. Results are presented explicitly for all the dipole - dipole dispersion energy coefficients and for the triple - dipole coefficients for all three -body interactions involving the H$_2$CO, CH$_3$CHO, (CH$_3$)$_2$CO, CH$_3$NH$_2$, (CH$_3$)$_2$ NH, and (CH$_3$)$_3$N molecules. The estimated errors in the two - body and three - coefficients are 1% and 1-2%

Computational and Spectroscopic Studies of Re(I) Bipyridyl Complexes Containing 2,6-Dimethylphenylisocyanide (CNx) Ligand.

Density Functional Theory (DFT) calculations produce optimized geometries of the complexes [Re(CO)3(bpy)Cl] (1), [Re(CO)3(bpy)(py)](CF3SO3) (2), [Re(CO)3(bpy)(CNx)](CF3SO3) (3), and [Re(CO)(bpy)(CNx)3](CF3SO3) (4), where bpy = 2,2'-bipyridine, py = pyridine, and CNx = 2,6-dimethylphenylisocyanide in their ground and lowest-lying triplet states. The ground-state optimized geometry for the cation of [Re(CO)3(bpy)(CNx)](CF3SO3) (3) results in a Re-C (CNx) bond length of 2.10 Å, a Re-C (CO) bond length trans to CNx of 2.01 Å, and a Re-C (CO) bond length cis to CNx of 1.96 Å which compares favorably to the single-crystal analysis of a Re-C (CNx) bond length of 2.074(4) Å, a Re-C (CO) bond length trans to CNx of 1.971(4) Å, and Re-C (CO) bond length cis to CNx of 1.932(4) Å. The majority of the singlet excited-state energies calculated using Time-dependent Density Functional Theory (TDDFT) and Conductor-like Polarizable Continuum Model (CPCM) are metal-ligand-to-ligand charge transfer (MLLCT) states and are in good agreement with the UV-vis spectral energies for the complexes in ethanol. The complexes exhibit emission both at room temperature and at 77 K except 4 which is only emissive at 77 K. The 77 K emission lifetimes range from 3.9 μs for 1 to 8.8 μs for 3. The emissive lowest-lying triplet state is a (3)MLLCT state for complexes 1-3 but a triplet ligand-to-metal charge transfer ((3)LMCT) state for complex 4. The electronic, electrochemical, thermodynamic, HOMO-LUMO, and emitting-state energy gaps as well as the emission lifetimes increase in the order 1 < 2 < 3. A (3)d-d excited- state, which is located above the (3)LMCT state, accounts for the loss of room-temperature emission for complex 4.

Structure of Excited-State Transitions of Individual Semiconductor Nanocrystals Probed by Photoluminescence Excitation Spectroscopy

We perform for the first time photoluminescence excitation (PLE) studies of individual nanocrystals (NCs) that reveal the structure of excited-state transitions not obscured by ensemble averaging. Single-NC PLE spectra strongly deviate from a traditional idealized picture of sharp, quasiatomic resonances. We detect only a few relatively narrow transitions (3-4 meV) at the band edge, while at higher spectral energies, we observe a broad structureless feature separated from the band-edge peaks by a >50 meV "minigap." These observations can be rationalized by analyzing hole intraband relaxation behavior.

Wideband Reflectance Measures of the Ipsilateral Acoustic Stapedius Reflex Threshold

The purpose of this study was to develop a method for acoustically measuring the ipsilateral acoustic stapedius reflex threshold by using wideband shifts in energy reflectance and admittance. A group of 27 young adult subjects with normal hearing participated in the study. Contralateral reflex thresholds were first measured for a 4000 Hz activator tone (maximum level, 92 dB SPL), using a clinical method with a 226 Hz probe tone. Ipsilateral and contralateral reflex thresholds were then measured by using an experimental wideband reflectance and admittance system that used a band-filtered click (200 to 2000 Hz) as the probe stimulus, presented simultaneously with the 4000 Hz activator tone. Reflex thresholds for the wideband system were determined by using statistical tests of the magnitude of the reflex responses as well as their correlation with other reflex responses. Clinical and experimental reflex thresholds were obtained for 9 of the 27 subjects for all conditions. Clinical reflex thresholds were absent for 8 subjects for whom experimental reflex thresholds were present and were present for 5 subjects who had absent experimental reflex thresholds for one or more conditions. An additional 5 subjects had absent reflex thresholds in all conditions, consistent with the low maximum level of the activator. Wideband measures of contralateral reflex thresholds were approximately 3 dB lower than those obtained with the clinical system. The magnitudes of the group means of the reflex responses were similar for ipsilateral and contralateral stimulations. Wideband measures of reflectance and admittance may be used to estimate the ipsilateral acoustic stapedius reflex threshold by separating in frequency the spectral energies of the wideband probe stimulus from the activator stimulus. This technique holds promise for measuring reflex thresholds for individuals with absent reflex thresholds through the use of standard clinical methods.

Spectroscopic studies of metal high-k dielectrics: transition metal oxides and silicates, and complex rare earth/transition metal oxides

This paper uses X-ray absorption spectroscopy to the study of electronic structure of the transition metal oxides TiO2, ZrO2 and HfO2, Zr and Hf silicate alloys, and the complex oxides, GdScO3, DyScO3 and HfTiO4. Qualitative and quantitative differences are identified between dipole allowed intra-atomic transitions from core p-states to empty d*- and s*-states, and inter-atomic transitions from transition metal and oxide 1s states to O 2p* that are mixed with transition metal d*- and s*-states for transition metal oxides and silicate alloys. The complex oxide studies have focused on the O K1 edge spectra. Differences between the spectral peak energies of the lowest d*-features in the respective O K1 spectra are demonstrated to scale with optical band gap differences for TiO2, ZrO2 and HfO2, as well as the complex oxides providing important information relevant to applications of TM oxides as high-k gate dielectrics in advanced Si devices. This is demonstrated through scaling relationships between (i) conduction band offset energies between Si and the respective dielectrics, and the optical band gaps, and (ii) the optical band gaps, the conduction band offset energies, and the electron tunneling masses as functions of the atomic d-state energies of the transition metal atoms.

X-ray absorption spectra for transition metal high-κ dielectrics: Final state differences for intra- and inter-atomic transitions

This article applies x-ray absorption spectroscopy to a study of the electronic structure of the high-k gate dielectrics, TiO2, ZrO2, and HfO2. Qualitative and quantitative differences are identified between intra-atomic transitions such as the Zr 3p-state, M2,3 core state absorptions which terminate in TM 4d*- and 5s*-states, and inter-atomic transitions such as the Zr 1s- and O 1s-state K1 absorptions which terminate in Zr 4d*- and 5s*-states that are mixed with O atom 2p* states through nearest neighbor bonding interactions. Differences between the spectral peak energies of the lowest d*-features in the O K1 spectra are demonstrated to scale with optical band gap differences for TiO2, ZrO2, and HfO2, providing important information relevant to applications of TM oxides as high-κ gate dielectrics in advanced Si devices. This is demonstrated through additional scaling relationships between (i) conduction band offset energies between Si and the respective dielectrics, and the optical band gaps, and (ii) the conduction band offset energies, and the electron tunneling masses as well.

Matrix-Isolation in Cryogenic Water-Ices: Facile Generation, Storage, and Optical Spectroscopy of Aromatic Radical Cations

Radical cations of naphthalene and 4-methylpyrene have been generated for the first time in high conversion efficiencies in cryogenic water-ices at 15 K through vacuum ultraviolet photolysis. With these radical cations as probes it is shown that cryogenic water-ices at temperatures below 50 K are of good optical quality and inert matrices to isolate and study the electronic spectroscopic properties of neutral and ionic species in the wavelength region 250−900 nm. The spectral energies of guest-species in the cryogenic water-ices are closely comparable with those observed using rare-gas matrices, indicating similar host−guest interactions in rare-gas matrices and water-ices below 50 K. The radical cations are converted to the corresponding alcohols at temperatures higher than 50 K due to reactions between the host and ionized guest species. Thus, cryogenic water-ices are inert matrices that resemble and complement the rare-gas matrices in many aspects. Efficient ionization of organic molecules, such as PAHs studied here, in water-rich ices indicates that ionization-mediated processes play an important role in the evolution of cosmic ices that are exposed to ionizing radiation.

Heart Rate Variability as an Indicator of Outcome in Congenital Diaphragmatic Hernia With and Without ECMO Support

To examine differences in the spectral power content in neonates diagnosed with congenital diaphragmatic hernia (CDH) who survive or succumb. A case-series study design evaluated four neonates diagnosed with CDH, two of which were supported by extracorporeal membrane oxygenation (ECMO). The electrocardiogram signal was digitized at 1000 Hz and the Lomb periodogram was computed for the series of interbeat intervals. Neonates with CDH who survived had log total power values greater than 2. Those with CDH who did not survive had log total power less than 2, but generally exceeded 3 while they were supported by ECMO. Neonates who consistently displayed increasing total spectral energies had a better outcome than those whose spectral energies were low. Subjects who succumbed expressed the lowest values, suggesting that a frequency-based evaluation of HRV may be a sensitive prognosticator of outcome that requires further investigation.

Phase-sensitive spectroscopy of surface plasmons in individual metal nanostructures

We analyze both theoretically and experimentally the response of individual gold nanoparticles and nanoparticle dimers to a near-field excitation with broad-band radiation ~a femtosecond white-light continuum! delivered through a subwavelength aperture. Because of the coherent superposition of the field emitted by the aperture and the secondary field reemitted by the nanostructure, the signals detected in the far zone exhibit a pronounced dependence on the phase of plasmon oscillations excited in the nanostructure. This phase sensitivity allows us to accurately determine positions of plasmon resonances not distorted by dielectric losses in a metal. In the near-field extinction spectra of individual nanoparticles, the plasmon resonance is observed as the ‘‘zero-extinction’’ point in which the transition from constructive ~lower spectral energies! to destructive ~higher spectral energies! interference occurs. By using spatially selective near-field excitation with a femtosecond white-light continuum, we are able to detect slight asymmetries in dimers composed of nominally identical nanoparticles.

Analysis of S-Wave Attenuation in South-Central Alaska

The S -wave attenuation of the crust and upper mantle in south-central Alaska is determined from shear-wave spectral data of 60 earthquakes with magnitudes 3.0–6.3 recorded by a network of 22 strong motion stations in Anchorage. The direct S -wave attenuation ( \(Q_{{\beta}}^{-1}\) ) and coda attenuation ( \(Q_{\mathrm{c}}^{-1}\) ) were estimated by the generalized inversion and coda decay methods, respectively. The scattering and intrinsic attenuation coefficients, η s and η i , were determined by applying a multiple scattering model to the variation of integrated spectral energy with hypocentral distance for three consecutive lapse time windows (0–15, 15–30, 30–45 sec), starting from the onset of the S -wave arrival. In order to obtain the dependence of attenuation coefficients on frequency, the spectral energies over an octave bandwidth for three central frequencies, 1.0, 3.0, and 6.0 Hz, were calculated. The results show the following: (1) \(Q_{{\beta}}^{-1}\) is higher than \(Q_{\mathrm{c}}^{-1}\) for frequencies ( f ) between 0.6 and 3 Hz and less for f > 3.0 Hz; (2) for 1.0 Hz ≤ f ≤ 6.0 Hz, η i increases and η s decreases gradually with f; (3) the seismic albedo, B 0 = η s /(η s + η i ), of the medium decreases from 0.48 to 0.15 with increasing frequency for 1.0 Hz ≤ f ≤ 6.0 Hz; (4) the total attenuation coefficient, \(L_{\mathrm{e}}^{-1}={\eta}_{\mathrm{i}}+{\eta}_{\mathrm{s}}\) , is nearly frequency independent; (5) at low frequencies (∼1.0 Hz) the scattering attenuation ( \(Q_{\mathrm{s}}^{-1}=k^{-1}{\eta}_{\mathrm{s}}\) , k is the wavenumber) is comparable to the intrinsic attenuation ( \(Q_{\mathrm{i}}^{-1}=k^{-1}{\eta}_{\mathrm{i}}\) ), while for high frequencies ( f ≥ 3.0 Hz) \(Q_{\mathrm{i}}^{-1}\) dominates the attenuation process compared to \(Q_{\mathrm{s}}^{-1}\) ; and (6) at 1.0 Hz, \(Q_{\mathrm{c}}^{-1}\) lies in between \(Q_{\mathrm{i}}^{-1}\) and total attenuation, \(Q_{\mathrm{t}}^{-1}\) , but is closer to \(Q_{\mathrm{t}}^{-1}\) for frequencies higher than 1.0 Hz.

Acoustic Emission Characterization of Damage in Hybrid Fiber-Reinforced Polymer Rods

The development and testing of a pseudoductile hybrid fiber-reinforced polymer rod consisting of glass and carbon fibers is described. Two different hybrid forms were considered for the development of the hybrid rods, including (1) random dispersion of the glass and carbon fibers; and (2) a core-shell design with a glass fiber core protected by the carbon fiber shells. The mechanical properties of the hybrids were evaluated through uniaxial tension tests. The hybrid rods developed by way of the core-shell approach exhibited pseudoductile properties that included a yield point of 1,153 MPa and an ultimate strength of 1,191 MPa, with a strain of 3.5% at ultimate. The experimental program also included damage assessment by way of a single-channel fiber optic acoustic emission (AE) sensor. The acoustic emission technique was employed for real-time determination of the progressive damage due to rupture of fibers. Moreover, the spectral energies of the frequency spectrum from the AE signals were employed for assessment of the state of damage in the rods. The spectral energies distinguish between the carbon and glass fiber ruptures. This differentiation of signals provides information about the condition and the state of health of the hybrid rod.

Boundary Protection - A Power Line Protection Technique Based on Fault Generated High Frequency Currents

This paper presents the boundary protection (BP) technique, a protection technique based on the detection of fault generated high frequency current signals. In the boundary protection technique, the power system busbars are used as the boundaries of protection zones. Advanced signal processing technique is applied to analyse the captured transient signals and extract high frequency signals at the desired bands. Then the spectral energies at different frequency bands are processed to distinguish between internal and external faults. In this paper, the principles of the boundary protection technique and the relay design are described. The application case studies of the technique under various system situations are presented. It is shown that the technique has demonstrated the potential to provide a solution for realising a camplete power line protection scheme without the need of a cammunicatian link.

Comparison of the analytical performance of high-powered, microwave-induced air plasma and nitrogen plasma atomic emission spectrometry

A high-powered, microwave-induced air plasma (air-MIP) generated by an Okamoto cavity at atmospheric pressure was investigated and compared with a nitrogen plasma (N2-MIP) generated by the same cavity under the same conditions. The analytical performance of the plasmas was evaluated and compared with regard to excitation temperatures, electron-number densities, detection limits and tolerance to direct organic solvent introduction. The excitation temperature of the air-MIP was in the range of 4150–4750 K when the microwave power varied from 0.8 to 1.3 kW, which was 300–400 K lower than those of the N2-MIP. The electron number densities in the air-MIP ranged from 1 to 6 × 1014 cm−3, which was about one order of magnitude higher than those of the N2-MIP. The detection limits for 17 elements with a total of 38 atomic and ionic lines were determined. For most of the spectral lines with lower excitation energies, the detection limits obtained from the air-MIP were comparable with those obtained from the N2-MIP, whereas for those spectral lines with higher excitation energies, the detection limits in the air-MIP were much poorer than those in the N2-MIP. Preliminary experiments showed that the air-MIP was highly tolerant to direct organic solvent introduction.

Analysis of aluminosilicate particles in biological matrices using histochemistry and X-ray microanalysis.

Aluminosilicates are a group of ubiquitous environmental particles that, in some cases, have been implicated in human disease. Characterisation of aluminosilicates in tissue samples requires, first, their in situ identification and, secondly, analysis of their aluminium and silicon content or, at least, aluminium:silicon ratio. Here, histochemical staining, microscopy and X-ray microanalysis were investigated as potential methods for the detection of aluminosilicates in biological samples. In contrast to aluminium phosphate or hydroxide, aluminosilicates were refractory to histochemical staining for aluminium. However, using electron microscopy, back scattered electrons allowed identification of aluminosilicates in tissue-like (gelatine) sections. X-ray microanalysis, using conventional peak:background ratios, did not provide a sufficiently accurate assessment of the aluminium content of various standard aluminosilicates to allow their identification. However, the similar spectral energies of aluminium and silicon allowed spectral peak heights to be directly compared and, using simple standards, aluminium:silicon ratios were found for a range of reference particles. Application of this technique should allow the localisation and identification of aluminosilicates in biological samples.

On the Spectral Energy Dependence of Gamma‐Ray Burst Variability

The variable activity of a gamma-ray burst (GRB) source is thought to be correlated with its absolute peak luminosity—a relation that, if confirmed, can be used to derive an independent estimate of the redshift of a GRB. We find that bursts with highly variable light curves have greater νFν spectral peak energies when we transform these energies to the cosmological rest frame using the redshift estimates derived either from optical spectral features or from the luminosity-variability distance indicator itself. This positive correlation between peak energy and variability spans ≈2 orders of magnitude and appears to accommodate GRB 980425, lending credibility to the association of this burst with SN 1998bw. The existence of such a correlation not only provides an interesting clue to the nature of this luminosity indicator but potentially reinforces the validity of the redshift estimates derived from this method. It also implies that the rest-frame GRB peak energy is correlated with the intrinsic luminosity of the burst, as has been suggested in the past as an explanation of the observed hardness-intensity correlation in GRBs.

Beam models for gamma-ray bursts sources: outflow structure, kinematics and emission mechanisms

The variety of γ-ray burst phenomenology could be largely attributable to differences in the opening angle of an isotropic outflow or to a standard type of event viewed from different orientations. Motivated by this currently popular idea, we study the effects of varying the energy per unit solid angle in the unsteady expelled outflow by an increase either in the bulk Lorentz factor or in the baryon loading. We apply these models to interpret the observed correlations between variability, luminosity and spectral peak energy and find that while the latter scenario fails to provide a good description, bursts produced by collisions between similar mass shells but with increasingly large Lorentz factors are both more variable and have larger peak spectral energies. We present detailed internal shock calculations confirming this interpretation and discuss the roles various timescales, radii and the optical thickness of the wind play in determining this wide range of behaviors. Finally, we discuss the variety of scenarios in which large variations of the source expansion velocity are naturally expected.

Discussion

The production of sibilant sounds such as /s/ and /z/ involves precise placement of the tongue tip/blade near the alveolar ridge to produce a constriction in the oral cavity. The constriction consists of a tapered channel formed by a central groove in the tongue and lateral tongue contact with the maxillary dentition. Expired airflow is funneled through the constriction to produce turbulence (ie, audible noise). Acoustically, the production of /s/ is characterized by spectral energies beginning in the region of 4 to 5 kHz. 1 Kent R Read C The acoustic analysis of speech. in: Singular Publishing, San Diego, CA1992: 92-94 Google Scholar Production of sibilant sounds such as /∫/ and /3/ involves a more posterior placement of the tongue near the palate to form a constriction. Because a longer resonating cavity exists anterior to the constriction, the production of /∫/ results in lower spectral energies beginning in the region of 2 to 3 kHz. The distinct acoustic characteristics of /s/ and /∫/ provide listeners with cues to alveolar and palatal places of articulation, respectively.

Quality measures in applications of image restoration.

We describe a new method for the estimation of image quality in image restoration applications. We demonstrate this technique on a simulated data set of fluorescent beads, in comparison with restoration by three different deconvolution methods. Both the number of iterations and a regularisation factor are varied to enforce changes in the resulting image quality. First, the data sets are directly compared by an accuracy measure. These values serve to validate the image quality descriptor, which is developed on the basis of optical information theory. This most general measure takes into account the spectral energies and the noise, weighted in a logarithmic fashion. It is demonstrated that this method is particularly helpful as a user-oriented method to control the output of iterative image restorations and to eliminate the guesswork in choosing a suitable number of iterations.