Asymmetric Absorption Research Articles

Numerical simulation of front lobe formation in laser-induced spark ignition of CH 4/air mixtures

Initial flame-kernel development in laser-induced spark ignition has been investigated numerically for methane/air mixtures. Laser energy absorbed by the spark has been simulated accounting for spatial distribution, especially, asymmetric absorption in the laser-beam direction. Results showed that the shock wave generated from the expansion of high-pressure initial kernel propagates rapidly having nearly spherical shape and leaving behind a high-temperature region. A torus-like shape flame kernel propagated radially and a front lobe was formed, which propagated back toward the laser source. These kernel structures were developed due to vortical motions generated by the interaction of pressure field and flow, which stems from the asymmetric deposition of laser energy in the direction of laser. The front lobe was separated from the torus for the lean mixtures, while the lobe and torus were connected for the stoichiometric mixture. The front lobe was extinguished at a later stage for the lean mixture at subatmospheric pressure. The calculated results agreed well with experimental observations.

Influence of dehydration effects on the optical spectra of H 4PVMo 11O 40 in the visible and near infrared range: intra- and intercenter optical transitions in the V–Mo cluster

UV–Vis–NIR diffuse reflectance spectra of heteropoly acids of the type of H 4PVMo 11O 40. xH 2O were recorded with improved spectroscopic equipment for different times on helium stream at room temperature. A broad asymmetric absorption band with a maximum at 740–780 nm in the visible range and a shoulder at 900–1000 nm in the near infrared range is identified. With increasing time on stream the intensity of this band grows appreciably: simultaneously, the water peaks at 1430 and 1925 nm decrease. Focusing on a single mixed-valence V 4+–Mo 6+-cluster, a model is suggested for the description of the experimental results. The model takes into account electron transfer, low symmetry crystal fields and vibronic interaction. The absorption band shape calculation is performed within the semiclassical limits. The band shape is shown to be determined both by the optical d–d transitions in V 4+ and Mo 5+ and the intervalence V 4+–Mo 6+ optical transition. Appreciable changes in the band intensity are attributed to the increase in the electron transfer parameter due to water loss. Quite good agreement with the experiment was found for a reasonable set of transfer and vibronic parameters.

Observation of strong exciton–photon coupling in an organic microcavity

An optical organic semiconductor microcavity showing the strong coupling regime was prepared. The dye 1,1 ′-diethyl-2,2 ′-cyanine (PIC) forming J-aggregates was used as optically active material in a planar λ/2-microcavity configuration. Stable thin layers of homogeneously distributed aggregates without any matrix were prepared by spin-coating of a specific salt of the dye [(PIC +) 2B 10H 10 2−]. Strong exciton–photon coupling was demonstrated by the anticrossing of exciton and photon modes. A Rabi-splitting of more than 50 meV was observed in transmission measurements. The asymmetric normal mode splitting could be explained by the asymmetric absorption spectrum of the J-aggregate.

Asymmetric Free-Free Absorption towards a Double Lobe of OQ 208

We present 1.66 GHz VSOP (VLBI Space Observatory Programme) observations of a GHz-peaked spectrum (GPS) source, OQ 208. Two compact lobes with a 10 pc separation in the NE-SW direction are identified with those observed with ground VLBI at higher frequencies. We find that the two lobes are highly asymmetric in terms of a flux density ratio NE:SW of ∼ 60:1 at 1.66 GHz, larger than that at higher frequencies. Both lobes show a steeply rising spectrum between 1.66 and 2.32 GHz, which implies free-free absorption by an ambient plasma, rather than synchrotron self-absorption. The absorption features show different peak frequencies, and indicate a larger optical depth towards the fainter SW lobe. This suggests the existence of an external absorber embedding the lobes. Hence, we can estimate the electron temperature, Te, and the electron density, ne, of the absorber. Considering the optical depth and X-ray luminosity, we derive the electron temperature and density in the ranges of 104 < Te < 6 × 107 K and 600 < ne < 7 × 105 cm−3, respectively. These properties coincide with those of NLR (Narrow Line Region). We point out that the NLR plasma affects evolution of radio galaxies, paticularly GPS sources.

VSOP observations of a GHZ-peaked spectrum source OQ 208

We present λ18 cm VSOP observations towards a GHz-peaked spectrum (GPS) source OQ 208. A double lobe seen by high-frequency ground VLBI observations is confirmed, although its brightness is more asymmetric at lower frequency. Comparing with images at higher frequency, we found asymmetric free-free absorption towards the double lobe. We can explain the spectral asymmetry by a slant jet axis against line of sight, which causes different optical depth in the ambient plasma.

Vibronic theory for the x-ray absorption spectrum ofCF4molecules

The C $1s$ x-ray absorption spectrum of ${\mathrm{CF}}_{4}$ molecules is theoretically investigated by calculating the line shape quantum mechanically based upon a vibronic model. It is found that the main peak with a broad width is attributed to the transition of the C $1s$ core electron to the antibonding valence orbitals which are coupled with the $\mathrm{C}\char22{}\mathrm{F}$ bond-stretching modes through the Jahn-Teller and the quasi-Jahn-Teller interactions. It is also clarified that the sharp asymmetric absorption line at the higher-energy side of the main band comes from the transition to the Rydberg states, where the interference effect between the Rydberg states and the vibronic state of the antibonding molecular orbitals leads to the asymmetric structure.

Discussion of the 1.04‐μm water ice absorption band in the Europa NIMS spectra and a new NIMS calibration

Measurements made by the Galileo Near Infrared Mapping Spectrometer (NIMS) have been used to calculate reflectance spectra for regions of Europa. Many of these spectra show features indicative of water ice. Others show asymmetric H2O absorption features suggesting hydrated non‐ice constituents such as hydrated sulfates and carbonates. Often, both types of features are present. However, some previously published NIMS spectra with strongly developed ice features did not show the expected 1.04‐μm ice absorption band. This discrepancy has led to questions regarding earlier interpretations of Europa's spectral features. Recent work reveals that unexpected changes have occurred in the spectral range covered by each NIMS detector and that an additional calibration is required. Preliminary calibration values are given. When this new calibration is applied, the 1.04‐μm feature emerges clearly in those spectra where it was expected but absent previously, thereby resolving the 1.04‐μm band discrepancy.

Neutrino-nucleon interactions in magnetized neutron-star matter: The effects of parity violation

We study neutrino-nucleon scattering and absorption in a dense, magnetized nuclear medium. These are the most important sources of neutrino opacity governing the cooling of a proto-neutron star in the first tens of seconds after its formation. Because the weak interaction is parity violating, the absorption and scattering cross sections depend asymmetrically on the directions of the neutrino momenta with respect to the magnetic field. We develop the moment formalism of neutrino transport in the presence of such asymmetric opacities and derive explicit expressions for the neutrino flux and other angular moments of the Boltzmann transport equation. For a given neutrino species, there is a drift flux of neutrinos along the magnetic field in addition to the usual diffusive flux. This drift flux depends on the deviation of the neutrino distribution function from thermal equilibrium. Hence, despite the fact that the neutrino cross sections are asymmetric throughout the star, the asymmetric neutrino flux can be generated only in the outer region of the proto-neutron star where the neutrino distribution deviates significantly from thermal equilibrium. The deviation from equilibrium is similarly altered by the asymmetric scattering and absorption, although its magnitude will still be quite small in the interior of themore » star. We clarify two reasons why previous studies have led to misleading results. First, inelasticity must be included in the phase space integrals in order to satisfy detail balance. Second, nucleon recoil must be included in order to find the leading order asymmetric cross sections correctly, even though it can be ignored to leading order to get the zero field opacities. In addition to the asymmetric absorption opacity arising from nucleon polarization, we also derive the contribution of the electron (or positron) ground state Landau level. For neutrinos of energy less than a few times the temperature, this is the dominant source of asymmetric opacity. Last, we discuss the implication of our result to the origin of pulsar kicks: in order to generate kick velocity of a few hundred kmhs{sup {minus}1} from asymmetric neutrino emission using the parity violation effect, the proto-neutron star must have a dipole magnetic field of at least 10{sup 15}{minus}10{sup 16} G. {copyright} {ital 1999} {ital The American Physical Society}« less

General Theory of the Energy Relations in Halls with Asymmetrical Absorption

In this paper we describe a method of calculation of the energy relations in halls where the existence of a non-uniform distribution of absorptive material in the room results in a non-diffuse sound field. The cases of halls used for concerts and speech have both been treated in order to derive new energy relations that yield known expressions when applied to a diffuse sound field. The importance of the initial reverberation time corresponding to the first portion of the decay has been verified showing that the main subjective parameters relating to the sound energy are influenced strongly by this portion, which is called the Early Decay Time if it is measured in the first 10 dB of the decay.

Integrated Absorbances in the 3.4 μ[CLC]m[/CLC] CH[TINF][CLC][ITAL]n[/ITAL][/CLC][/TINF] Band in Hydrogenated Amorphous Carbon

We have measured the integrated absorbance, A (in units of cm molecule−1), for the spectral components of the 3.4 μm CHn absorption band in hydrogenated amorphous carbon (HAC) for samples deposited at 300 and 77 K. For the asymmetric CH3 stretching band in HAC, A is found to be significantly weaker than that occurring in pure saturated aliphatic hydrocarbons, while the symmetric mode of this group has a comparable intensity in HAC and in pure hydrocarbons. This suggests that the relative strength of CH3 symmetric and asymmetric absorption components may be a useful diagnostic of CH3 bonding in interstellar carbonaceous materials. Our measured absorbances, together with observed optical depths in the 3.4 μm absorption band in the diffuse interstellar medium, suggest that N(C)/N(H) ~ 72-97 parts per million in HAC in diffuse clouds. We discuss the possibility that aromatic carbon particles with CHn edge and defect groups may be the source of both the 2175 A extinction bump and the 3.4 μm CHn absorption band.

Fano Resonances in Semiconductor Superlattices

We report the first observation of Fano resonances in biased semiconductor superlattices: The excitonic Wannier-Stark ladder transitions show asymmetric absorption due to the coupling to the continua of lower transitions. In contrast to other known examples of Fano resonances, the Fano coupling can be continuously tuned in this system by changing the static field across the superlattice. The line shapes and their coupling dependence are in excellent agreement with theory. We also investigate the dephasing dynamics of the resonances and observe an increase in dephasing with increasing Fano coupling.

Deposition Temperature Dependence of Optical Gap and Coloration Efficiency Spectrum in Electrochromic Tungsten Oxide Films

Optical gaps and electrochromic efficiencies of sputtered tungsten oxide films are studied by focusing attention on the cluster size of the film. The cluster consists of O‐W‐O network with terminal W=O bonds on its boundary. The quantity of W=O bonds increased with the surface area of the clusters. Raman scattering bands of the O‐W‐O and W=O are observed between 500 and 1100 cm−1. These characteristic Raman scattering bands of the film are well reproduced by the combination of four Gaussian shaped Raman bands of the O‐W‐O and W=O modes. The ratio of integrated Raman scattering intensities (W=O/O‐W‐O) of these modes are employed as the measure of the cluster size. The cluster is found to increase in size with the elevation of the film deposition temperature. Optical gap was observed to increase inversely proportional to the cluster size of the film. This cluster size dependence appears to have the same origin assigned in nanocrysallites. The electrochemical reduction of the film produces a broad asymmetric polaron absorption band in the optical region from near IR to visible. The coloration efficiency spectrum of this absorption band becomes higher with the cluster size, and its maximum position shifts to a lower energy side. This behavior is explained in terms of a polaron‐polaron interaction and a polaron confinement in the cluster.

Effect of charge interactions on the carboxylate vibrational stretching frequency in c-type cytochromes investigated by continuum electrostatic calculations and FTIR spectroscopy

The FTIR spectra of the asymmetric carboxylate absorption region of three c-type cytochromes—namely horse heart, yeast and bonito cytochromes c—as well as continuum electrostatic calculations performed on their respective protein matrices, show that these combined methods can target specific protein regions and yield pertinent protein charge information that correlates with the observed spectral data. Deconvolution of the IR carboxylate stretch frequency region (1525–1675 cm −1) in the three cytochromes yield different ν oco a distributions. In the case of the bonito cytochrome c carboxylates, two ν oco a populations are clearly distinguishable in the deconvoluted spectra—which is not the case for the more complex ν oco a deconvolutions of the other two cytochromes. The frequency distributions of the calculated potentials are consistent with the experimental observations and we conclude that the IR carboxylate absorption in proteins can be modified by the electrostatic environment.

Vibrations of alkali cations in borate glasses from molecular orbital calculations

We have performed ab initio molecular orbital calculations on a cluster of atoms modelling the medium-range structure in Na 2O·3B 2O 3 glass at the Hartree–Fock/3-21G(*) level. It is shown that the optimized structural parameters are in good agreement with the observed ones. Using the optimized cluster, we have further carried out a vibrational analysis to interpret the far infrared vibrational spectra observed for sodium borate glasses. The frequency calculations of the cluster successfully reproduce the observed far infrared asymmetric absorption profiles which can be deconvoluted into two broad bands. According to the normal coordinates calculated, the two absorption bands are attributed to two different types of vibrations of alkali cations in their anionic sites whose vector displacements are almost perpendicular to each other.

Tomographic Reconstruction of 2-D Absorption Coefficient Distributions from a Limited Set of Infrared Absorption Data

The results of infrared absorption experiments, performed on a steady, nonreacting flow of methane and argon to test a new inversion method for tomographic reconstruction of spatially nonuniform distributions, are presented. The absorption coefficient distributions to be reconstructed are expressed as a weighted sum of Karhunen-Loève eigenfunctions, formed by applying the Karhunen-Loève procedure to a training set containing a priori information regarding the distributions to be reconstructed. Reconstruction of asymmetric absorption coefficient distributions is accomplished by utilizing a set of laser absorption measurements obtained using an infrared helium neon laser. Issues pertinent to the practical application of the new inversion method, such as the construction of a training set using probe sampling, the effect of experimental error, and the precise modeling of laser absoroption paths, are discussed. Reconstruction of absorption coefficient distributions from only 42 laser absorption measurements typically achieve normalized errors of 10% or less over 80% of the domain.

Controlling the sign of quantum interference by tunnelling from quantum wells

The sign of the interference (constructive or destructive) between quantum-mechanical paths depends on the phase difference between the paths. In the Fano effect1 two optical paths from the ground state of a system — one direct and one mediated by a resonance — to a state in an energy continuum interfere to produce an asymmetric absorption spectrum that falls to zero near the absorption maximum. Zero absorption occurs as the wavelength is scanned across the resonance, at a photon energy corresponding to a 180 ° phase difference between the paths. Similar interference effects occur when two absorption paths are mediated by two different states, and they provide the basis for lasers that operate without a population inversion2,3,4,5,6,7. Here we report the control, by quantum mechanical tunnelling, of interference in optical absorption. The two intermediate states are resonances that arise from the mixing of the states in two adjacent semiconductors quantum wells, which are broadened by tunnelling into the same energy continuum through an ultra-thin potential-energy barrier. Inverting the direction of tunnelling by reversing the position of the barrier with respect to the two quantum wells changes the interference from destructive to constructive, as predicted theoretically. This effect might provide a way to make semiconductor lasers without population inversion8.

Characterization of the Bis(azido)(meso-tetraphenylporphinato)ferrate(III) Anion. An Unusual Spin-Equilibrium System.

The complex anion bis(azido)(tetraphenylporphinato)ferrate(III) has been synthesized and characterized by variable-temperature X-ray structure determinations, powder and single-crystal EPR, IR, and Mössbauer spectroscopy, and magnetic susceptibility measurements. The synthesis utilizes 18-crown-6 to solubilize sodium azide in the synthetic procedure. All physical data for [Na(18C6)(H(2)O)(2)][Fe(TPP)(N(3))(2)].2C(6)H(5)Cl are consistent with a thermal spin-equilibrium system: low spin (S = (1)/(2)) right harpoon over left harpoon high spin (S = (5)/(2)). Structure determinations at 130 and 293 K show equatorial and axial Fe-N bond elongation at 293 K. Fe-N(p) = 1.9991(11) Å, Fe-N(az) = 1.9734(14) Å at 130 K, and Fe-N(p) = 2.010(4) Å, Fe-N(az) = 1.998(2) Å at 293 K. The EPR g values for a powder sample at 4.2 K are 1.81, 2.18, and 2.70. A fit of the powder EPR spectrum at 4.2 K with a crystal field model that allows quartet and sextet admixtures suggests that the first sextet state is approximately 655 cm(-)(1) above the ground doublet. Single-crystal EPR data indicate that the largest g value occurs at an angle of 56 degrees from the porphyrin normal and at 35 and 81 degrees from the Fe-N(p) vectors. The asymmetric azide IR absorption bands at 2014 and 2036 cm(-)(1) can be assigned to low- and high-spin species, respectively, and display temperature-dependent intensities. The Mössbauer experiments reveal a gradual decrease in the quadrupole splitting as the temperature increases from 140 to 300 K. The magnetic susceptibility measurements show a gradual increase of &mgr;(eff) with temperature. Crystal data for [Na(18C6)(H(2)O)(2)][Fe(TPP)(N(3))(2)].2C(6)H(5)Cl (130 K): a = 11.417(2) Å, b = 12.371(4) Å, c = 12.628(2) Å, alpha = 64.30(2) degrees, beta = 77.18(3) degrees, gamma = 77.67(2) degrees, triclinic, space group P&onemacr;, Z = 1. Crystal data (293 K): a = 11.7652(12) Å, b = 12.6488(6) Å, c = 12.8608(13) Å, alpha = 62.02(2) degrees, beta = 75.996(7) degrees, gamma = 75.465(9) degrees.

SUPERHUMPS IN CATACLYSMIC BINARIES. XII. CR BOOTIS, A HELIUM DWARF NOVA

ABSTRACT CR Bootis is an enigmatic blue variable star with rapid photmetric variations and a spectrum dominated by helium. It consists of two white dwarfs in close orbit, with a probable underlying binary period of 1471 s. For years we have marveled at the star's large nightly variations - ramping up or down at a rate of ~0.1/hr. An intensive photometry campaign in 1996 showed that this variability is cyclic with a quasiperiod of about 19 hr, and demonstrated association of 14990 s photometric variations ("superhumps") with extended bright states ("superoutbursts"). During superoutburst, the 1490 s signal initially decreased with dot P = -2x 10-5 , but then stabilized at 1487.29±0.02 s after ~300-600 binary orbits. Spectroscopy reveals variably asymmetric absorption lines, with the asymmetry migrating on a probable period of 36 hr; this may be the period of accretion disk precession. Neither the helium composition, nor the degeneracy of the mass-losing component, nor the shortness of the period (all of the periods) seem to present any barrier to the star in being fully certificable as a bona fide dwarft nova. Stabilization of the superhump period at such a low value (1487.29 s) favors a model in which period changes arise from eccentricity changes rather than mean radius changes in the disk. This naturally explains why decreasing period and decreasing amplitude are strongly linked in the superhumps of dwarf novae.

Synthesis of silicon oxynitride by ion beam sputtering and the effects of nitrogen ion-assisted bombardment

Thin silicon oxynitride (SiO x N y ) films were synthesised without substrate heating by means of N 2 + ion-beam sputtering of a silicon nitride target at an energy of 1000 eV in a N 2 and O 2 ambient with and without 200 eV N 2 + ion assistance. Unassisted films were deposited in a controlled O 2 partial pressure ranging from ambient to 5.0 × 10 −3 Pa whereas assisted films were deposited at a fixed O 2 partial pressure of 1.0 × 10 −3 Pa. The O/(O+N) atomic fraction and the SiO x N y asymmetric stretch mode IR absorption peak wavenumber of unassisted films increased almost linearly with increasing O 2 partial pressure, from 0.2 to 1.0 and 860 cm −1 to 1050 cm −1, respectively, while their refractive indices decreased from 1.92 to 1.46. The behaviour of the SiO x N y film refractive index with the SiO 2 fraction has been compared to that predicted by Drude, Lorentz-Lorenz and Bruggeman models under the assumption that the film is a mixture of SiO 2 and Si 3N 4 phases. For a fixed O 2 partial pressure, the O content of the N 2 + ion-assisted films increased with an increase in the N + ion to Si atom arrival ratio from 0 to 3. This increase in O content correlate with changes in the film refractive index and SiO x N y asymmetric stretch mode absorption peak position, from 1.56 to 1.43 and 1014 cm −1 to 1054 cm −1, respectively, indicating that the O/N atomic ratio increases with increasing N + ion to Si atom ratio until film properties consistent with stoichiometric SiO 2 are obtained.

The gas phase infrared spectrum of HCl complexed with dimethyl ether revisited: assignment of the fundamental transition from a jet-cooled experiment

Preliminary IR spectroscopic observation of the ν s band of complexed HCl in a supersonic jet expansion of dimethyl ether-hydrogen chloride seeded with argon is presented. It is characterized by a structureless, asymmetric absorption, 25 cm −1 full-width at half-maximum, centered at 2460 cm −1. Its assignment to the fundamental transition agrees with the assignment initially proposed by Lassègues and Huong [Chem. Phys. Lett. 17 (1972) 444] and allows a re-examination in more detail of the gas cell data for both DME:HCl and DCl dimers.