Spectroscopic ellipsometry of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)

The optical functions of PTCDA have been determined by spectroscopic ellipsometry. The samples were prepared by thermal evaporation of PTCDA on glass substrates. The influence of the substrate temperature on the properties of PTCDA films has also been investigated. The lowest substrate temperature investigated here was room temperature. At room temperature and higher substrate temperatures, we have not observed ordering of the evaporated films, which would result in high anisotropy, as reported for PTCDA films grown by organic molecular-beam deposition at very low temperatures (so-called quasi-epitaxial growth). Therefore, the samples have been assumed to be isotropic. The obtained imaginary part of the dielectric function exhibits the expected features: a low-magnitude peak around 370 nm and a higher magnitude, two-peak structure in the 400–600 nm spectral region. PACS 78.20.Ci; 78.66.Qn     

无机-有机杂化非线性光学晶体材料LAP和d-LAP晶体红外吸收边计算

根据双原子谐振子模型近似,提出了估算分子晶体材料红外吸收边的理论方法,然后利用超分子量子化学从头算,计算了L-精氨酸一水磷酸盐(LAP)晶体和氘化L-精氨酸一水磷酸盐(d-LAP)晶体单元超分子的红外振动光谱,其中LAP晶体超分子计算值与晶体红外光谱实验值吻合,表明超分子计算能有效地模拟无机-有机杂化非线性光学晶体的红外振动光谱。在归属了超分子重要基频线后,分析其红外强度,并估算了泛频频率。根据我们提出的方法,估算了这两种晶体的红外吸收边,结果与实验值较吻合。表明我们建议的理论方法是合理的。此外,通过计算表明,如果非线性光学晶体材料的红外吸收边是由与活泼H有关的伸缩振动泛频频率决定,那么活泼H的氘化是一种改善红外吸收边的有效途径。     

Magnetoacoustic Oscillations of a Plasma Containing Two Species of Ions

Numerical calculations of linear magnetoacoustic resonant phenomena in a plasma containing two species of ions have been made for a cylindrical plasma with a model which includes the effects of collisional damping and radial non-uniformities in temperature and number density. At sufficiently high temperatures two frequencies are predicted at which magnetoacoustic resonances for the first radial mode will occur. These are expected from considerations of the effects of the ion-ion hybrid resonance.     

X-ray diagnosis of the pressure induced Mott nonmetal-metal transition

The evolution of the K-edge x-ray absorption near-edge spectroscopy (XANES) spectrum is investigated for an aluminum plasma expanding from the solid density down to 0.5 g/cm{3}, with temperatures lying from 5 down to 2 eV. The dense plasma is generated by nanosecond laser-induced shock compression. These conditions correspond to the density-temperature region where a metal-nonmetal transition occurs as the density decreases. This transition is directly observed in XANES spectra measurements through the progressive formation of a preedge structure for densities around 1.6 g/cm{3}. Ab initio calculations based on density functional theory and a jellium model have been efficiently tested through direct comparison with the experimental measurements and show that this preedge corresponds to the relocalization of the 3p atomic orbital as the system evolves from a dense plasma toward a partially ionized atomic fluid.     

Cuspidal edges for elastic wave surfaces for cubic crystals

The paper deals with a detailed numerical study of the sections of the inverse and ray velocity surfaces for cubic crystals. The figures for the sections of the inverse and ray surfaces by the (001) and (110) planes have been plotted for over 65 crystals and from these, the nature of the cuspidal edges has been discussed. Typical graphs of the inverse and ray surfaces have been given. The parameters characterising the dimensions of the cusps have been tabulated. It is shown that the A-15 compounds exhibit very unusual and interesting wave surfaces at temperatures below superconducting critical temperatures.     

Role of symmetry in the transport properties of graphene nanoribbons under bias

The intrinsic transport properties of zigzag graphene nanoribbons (ZGNRs) are investigated using first-principles calculations. It is found that although all ZGNRs have similar metallic band structure, they show distinctly different transport behaviors under bias voltages, depending on whether they are mirror symmetric with respect to the midplane between two edges. Asymmetric ZGNRs behave as conventional conductors with linear current-voltage dependence, while symmetric ZGNRs exhibit unexpected very small currents with the presence of a conductance gap around the Fermi level. This difference is revealed to arise from different coupling between the conducting subbands around the Fermi level, which is dependent on the symmetry of the systems.     

Field evaporation end form at 4.2 K

Field evaporation end forms of tungsten and iridium have been observed at several temperatures. At 21 K and above, the surfaces contain atoms in metastable positions which give rise to decorated zone lines, double plane edges and superstructures, but at 4.2 K these are scarcely observed. These results imply that at the higher temperatures an edge atom may migrate to a surface site before it evaporates, while at 4.2 K the edge atom may evaporate without surface migration. A considerable part of the migration energy is probably due to a field induced dipole-dipole repulsion.     

First observation of natural circular dichroism spectra in the extreme ultraviolet region using a polarizing undulator‐based optical system and its polarization characteristics

Natural circular dichroism (CD) spectra in the extreme ultraviolet (EUV) region down to a wavelength of 80 nm have been observed for the first time, using an alanine thin film deposited on sodium salicylate coated glass as a sample. Calibrated EUV‐CD spectra of l ‐alanine exhibited a large negative peak at around 120 nm and a positive CD signal below 90 nm, which were roughly predicted by theoretical calculations. A CD measurement system with an Onuki‐type polarizing undulator was used to obtain the EUV‐CD spectra. This CD system, the development of which took five years, can be used to observe even weak natural CD spectra. The polarization characteristics of this system were also evaluated in order to calibrate the recorded CD spectra.     

Current Neutralization and Charge Transport of an REB in Low-Pressure Argon

A numerical study of current neutralization and its effect on charge transport of a 0.175-MeV 6-kA 30-ns relativistic electron beam (REB) in argon at low pressure of 10-2-1 torr is presented. Contribution of two-stream instabilities to plasma conductivity is included in the calculations. The calculations show that energy coupled from beam to plasma is about 1-3 percent in the pressure range 0.1-1 torr. Though the two-stream contribution influences the time-dependent electron temperature and density around 0.1 torr, its effect on beam transport has been found to be only marginal. The results indicate that a maximum charge transport of 76 percent occurs around 0.08 torr when current neutralization is negligible. At 1-torr pressure, 75-percent current neutralization occurs and the charge transport is reduced to 40 percent. The numerical results have been compared with experiment and show good agreement in the pressure range 0.2-1 torr.     

Domain walls in the (Ga,Mn)as diluted magnetic semiconductor

We report experimental and theoretical studies of magnetic domain walls in an in-plane magnetized (Ga,Mn)As dilute moment ferromagnetic semiconductor. Our high-resolution electron holography technique provides direct images of domain wall magnetization profiles. The experiments are interpreted based on microscopic calculations of the micromagnetic parameters and Landau-Lifshitz-Gilbert simulations. We find that the competition of uniaxial and biaxial magnetocrystalline anisotropies in the film is directly reflected in orientation dependent wall widths, ranging from approximately 40 to 120 nm. The domain walls are of the Néel type and evolve from near-90 degrees walls at low temperatures to large angle [11[over ]0]-oriented walls and small angle [110]-oriented walls at higher temperatures.     

Ion population fraction calculations using improved screened hydrogenic model with l-splitting

Ion population fraction(IPF) calculations are very important to understand the radiative spectrum emitted from the hot dense matter. IPF calculations require detailed knowledge of all the ions and correlation interactions between the electrons of an ion which are present in a plasma environment. The average atom models, e.g., screened hydrogenic model with l-splitting(SHML), now have the capabilities for such calculations and are becoming more popular for in line plasma calculations. In our previous work [Ali A, Shabbir Naz G, Shahzad M S, Kouser R, Rehman A and Nasim M H 2018 High Energy Density Phys. 26 48], we have improved the continuum lowering model and included the exchange and correlation effects in SHML. This study presents the calculation of IPF using classical theory of fluctuation for our improved screened hydrogenic model with l-splitting(I-SHML) under local thermodynamic equilibrium conditions for iron and aluminum plasma over a wide range of densities and temperatures. We have compared our results with other models and have found a very good agreement among them.     

Calculation of radiative heat transfer in SF6 arcplasmas

Radiation transport in SF₆ are plasmas have been treated for pressures of 0.101, 0.5, and 1.0 MPa. We have investigated the use of an approximate method of partial characteristics. We have calculated spectral absorptivities for SF₆ plasmas for temperatures from 300 to 35 000 K, and have used these absorptivities to calculate the two functions designated ΔI and ΔSim. These functions are integrals over all radiation frequencies for given line segments with a linear variation of temperature along the line. The ΔI and ΔSim values are functions of the end temperatures and the length of the line, and are used to evaluate radiation transfer from line segments in the plasma where the temperature variation for each line segment is approximated as being linear. The validity of this method of partial characteristics has been demonstrated by comparing exact calculations with the approximate calculations to evaluate radiation intensities, radiation fluxes, and divergence of radiation fluxes for specified temperature profiles. The method of partial characteristics has been applied to one- and three-dimensional calculations of radiative heat transfer in simplified temperature profiles. Agreement up to 10% is obtained with exact calculations, but with a large reduction of computation time     

Self-Organization Phenomena in a Cryogenic Gas Discharge Plasma: Formation of a Nanoparticle Cloud and Dust–Acoustic Waves

The dusty plasma structures in a glow discharge of helium in a tube cooled by superfluid helium at a temperature of 1.6 K and higher have been studied experimentally. The bimodal dust plasma formed by clouds of polydisperse cerium dioxide particles and polymer nanoparticles has been analyzed. We have observed wave oscillations in the cloud of polymer nanoparticles (with a size up to 100 nm), which existed in a narrow temperature range from 1.6 to 2.17 K. Vortices have been observed in the dusty plasma structures at helium temperatures.     

Influence of atom-atom collisions on the collisional-radiative ionization and recombination coefficients of helium plasmas

Coefficients for volume recombination and ionization have been calculated for a dense helium plasma of low degree of ionization. The calculations are based on a collisional-radiative model in which electron-atom, electron-electron-ion, atom-atom, and electron-atom-ion collisions intervene. Molecular species such as He ₂ ^* and He ₂ ⁺ have not been taken into account. The essential results are: At low temperatures and high neutral gas densities the recombination coefficient is proportional to the number density of neutral helium atoms. At high temperatures the presence of neutral particles practically does not influence the recombination process compared to pure ion-electron-electron recombination. At high neutral particle densities, high atom temperatures and low electron densities the ionization process is mainly due to atom-atom collisions. In this point our calculations are in relatively good agreement with recent shock tube measurements of Kalra and Measures (Phys. Fluids14, 2544 (1971)). It is emphasized that the simple two-step model for ionization by shock waves in the noble gases should be replaced by a more general collisional-radiative model in which the atomic level structure intervenes in more detail.     

Ferromagnetism in the Hubbard model on fcc-type lattices

The Hubbard model on fcc-type lattices is studied in the dynamical mean-field theory of infinite spatial dimensions. At intermediate interaction strength finite temperature Quantum Monte Carlo calculations yield a second order phase transition to a highly polarized, metallic ferromagnetic state. The Curie temperatures are calculated as a function of electronic density and interaction strength. A necessary condition for ferromagnetism is a density of state with large spectral weight near one of the band edges. Received: 6 August 1997 / Accepted: 20 August 1997     

A comprehensive approach to the free vibration analysis of rectangular plates by use of the method of superposition

The authors have found the above techniques to constitute a powerful means for solving rectangular plate problems. At the time of writing, solutions for plates with two adjacent simply supported edges and two adjacent free edges have been obtained. The first 20 eigen-values for plates with all edges clamped have also been determined for a full range of aspect ratio and they are shown to be accurate to within less than one half of one percent. It will be appreciated that solutions for any combination of clamped-simply supported edge conditions can easily be obtained from the all-clamped solution by simply deleting appropriate solutions from the all-clamped combination. In Figure 2 contour lines for first mode vibration of a plate with two adjacent clamped and two adjacent simply supported edges is presented. The higher density of the contour lines along the simply supported edges will be noted.The method of superposition is currently being used by the authors to good advantage to obtain solutions of any desired degree of accuracy to all of the problems discussed. It is found to be easily utilized and unlike more complicated methods is readily comprehensible to the practicing engineer. Eigenvalues for all modes, aspect ratios, and boundary conditions are readily obtained. Modal shapes are expressed in terms of familiar analytic functions. Results of these studies will be made available in future publications.     

Energy gaps in graphene nanoribbons

Based on a first-principles approach, we present scaling rules for the band gaps of graphene nanoribbons (GNRs) as a function of their widths. The GNRs considered have either armchair or zigzag shaped edges on both sides with hydrogen passivation. Both varieties of ribbons are shown to have band gaps. This differs from the results of simple tight-binding calculations or solutions of the Dirac's equation based on them. Our ab initio calculations show that the origin of energy gaps for GNRs with armchair shaped edges arises from both quantum confinement and the crucial effect of the edges. For GNRs with zigzag shaped edges, gaps appear because of a staggered sublattice potential on the hexagonal lattice due to edge magnetization. The rich gap structure for ribbons with armchair shaped edges is further obtained analytically including edge effects. These results reproduce our ab initio calculation results very well.     

Assessing the Maturity of Crude Petroleum Oils Using Total Synchronous Fluorescence Scan Spectra

There have been many applications of fluorescence methods for the analysis of crude petroleum oils down through the years. However, none of these studies has yielded a robust qualitative or quantitative method for quantifying the chemical composition, or assessing the maturity of crude oils. Simple fluorescence parameters such as lifetime, intensity, and intensity ratios do not correlate well with chemical composition particularly for medium weight crude oils [A. G. Ryder, T. J. Glynn, and M. Feely (2003). Proc. SPIE-Int. Soc. Opt. Eng. 4876, 1188-1195.]. A better approach may be to use the Total Synchronous Fluorescence Scan (TSFS) method to fully interrogate the complex chemical composition of the oils [D. Patra and A. K. Mishra (2002). Anal. Bioanal. Chem. 373, 304-309.]. We present TSFS spectra from 18 crude petroleum oils of varying composition, sourced from around the world. The TSFS plots of these oils are very complex, with the contours being spread over the full 250-700 nm wavelength range (lambda(ex)) and 40-200 nm wavelength interval (delta lambda) sampled. The 3-D contour maps tend to two contour concentrations one at lambda(em) < 300 nm, delta lambda = 120-200 nm, and a second near lambda(ex) approximately 380-400 nm, delta lambda = 40-60 nm. The first feature represents fluorescence emission originating mainly from energy transfer processes with the second, longer wavelength feature originating from fluorescence emission generated by a higher proportion of direct excitation as opposed to emission resulting from energy transfer. The topography of the 3D contour plots is therefore influenced by the balance between energy transfer and direct fluorescence emission, which is governed by the chemical composition of the crude oils. We discuss how the gross chemical composition affects TSFS spectra and how TSFS can be used to assess oil maturity with a view to developing quantitative methods.     

激光诱导Al表面等离子体温度的发射谱诊断

探测了脉冲能量0.3 J波长1064 nm的纳秒激光聚焦Al表面诱导的等离子体200-900 nm范围的发射谱.分析了线状谱的基本规律,根据谱线的强度,考虑到光谱仪与电荷耦合器件(charge coupled device, CCD)的探测效率,通过线性拟合给出了等离子体中Al Ⅰ、Al Ⅱ、N Ⅰ、O Ⅰ粒子的激发温度.根据谱线的半高宽计算了等离子体电子密度,进而计算了等离子中Al Ⅰ的电离温度.结果表明,在等离子体状态快速演化过程中,不同粒子的电离、激发与退激存在较大差异,Al Ⅱ、Al Ⅰ相对于N Ⅰ、O Ⅰ有较高的激发温度,并且等离子体中Al Ⅰ的电离温度高于所有粒子的激发温度.