Model for cometary grains to explain optical polarization

The observed optical polarizations for comets have been explained in past assuming cometary grains to be compact spheres, such that Mie theory could be applied to simulate the observed polarizations. However, recently other shapes like spheroids and then more realistic shapes like aggregates of monomers have been considered for cometary grains, to explain the observed polarizations. For this purpose T-matrix or DDA based light scattering technique was mostly used to simulate the observed polarizations. A number of authors have used T-matrix, DDA and various other techniques along with aggregate grain model to explain the polarizations of comets like 1P/Halley, C 1995/O1 Hale-Bopp, C/1990 K1 (Levy) and C/1996 B2 Hyakutake, etc. Recent STARDUST mission had suggested cometary grains to be mixtures of compact and porous aggregates. Accordingly, attempts have been made recently to reproduce the cometary polarization with mixtures of various compositions, shapes and porosity.The work presented here considers a model for cometary grains which contains (1) solid grains of pyroxene (silicate) and organic with various sizes of spheres, prolates and oblates and (2) aggregates with monomers of various sizes, with composition of pyroxene (silicate) and organic, having structures (shapes) defined by BCCA and BAM2 codes. It was found that the present model can explain the observed polarization data, especially the negative branch, for comet 1P/Halley at , more effectively as compared to other work done in past. Among the aggregates the BAM2 structure was found to play a key role, in deciding the cross-over angle and depth of negative polarization branch.     

The anomalous spectral dependence of polarization in comets

We have found an anomalous (negative) spectral dependence of polarization for split comet 73P/Schwassmann–Wachmann 3, i.e. polarization degree in the I filter is systematically less (4%) than that in the R filter for both B and C subnuclei of the comet. Earlier we revealed a similar effect for comets 21P/Giacobini–Zinner, C/1999 S4 (LINEAR), and 9P/Tempel 1. Together with comet C/1989 X1 (Austin), for which the similar spectral dependence of polarization was also observed, these comets form a group characterized by some peculiar properties of their dust. Since atypical organic composition is a characteristic feature for all these comets, the anomalous spectral dependence of polarization may indicate specifics of origin of these comets. Really, most of these comets (but not all) originated from the Kuiper Belt. Also, most (but not all) of these comets belong to the group of comets depleted in carbon species in the gas phase, which is typical for comets formed in the giant-planet area of the solar system. On the other hand, all of the comets with negative spectral gradient of polarization have the dust dominated by large particles and most of them belong to the Jupiter family. This may indicate that anomalous spectral dependence of their polarization is a result of their evolution. To identify the reason of anomalous polarization, systematic spectropolarimetric observations of Jupiter-family comets are necessary.     

A complete measurement of spin-observables for intermediate-energy inclusive quasielastic polarized proton scattering from C

The complete set of Wolfenstein parameters, the polarization, the asymmetry of scattering and the unpolarized double-differential cross section are presented for inclusive quasielastic proton scattering from ¹²C at a central momentum transfer of q = 1.9 fm⁻¹ and incident energies of 290 and 420 MeV. The spin observables D₀, D_x, D_y and D_z as well as the longitudinal-to-transverse ratio of spin-flip probabilities are extracted from the data. Across the quasielastic continuum, the experimental data is compared to the variations expected from a single-scattering Fermi-gas approximation using the free NN amplitudes. Medium effects are evident in the pronounced quenching of the polarization parameter relative to the free value.     

Rigorous reflectance performance analysis of Si3N4 self-suspended subwavelength gratings

In this paper, we make an investigation into the reflectance performance of the Si₃N₄ self-suspended subwavelength grating (SSG) via changing several parameters including the incident wavelength, the grating period, the grating thickness and the grating filling ratio. For TM polarization of incidence, numerical results by the rigorous coupled-wave method reveal that the reflectance spectrum of the Si₃N₄ SSG has a narrow linewidth (less than 1 nm), high peak reflectance (100%), and low side bands (less than 0.5%) simultaneously in the optical telecommunication wavelength region. By using a micro-electromechanical-systems actuator, it is believed that the Si₃N₄ SSG should serve as tunable optical filters in optical telecommunication systems.     

Toward an understanding of white-light generation in cubic media - polarization properties across the entire spectral range

We present an extensive investigation of the polarization properties of a femtosecond-laser-induced white-light (WL) continuum generated in a cubic crystal (CaF(2)). The WL spectrum and threshold energies have been examined for input polarizations with various degrees of ellipticity. For linear input polarization, the WL spectrum shows strong depolarization around the input wavelength, while the preservation of the input polarization is pronounced toward the blue spectral region. The observed depolarization effect has been elucidated in the framework of current models for WL generation.     

卡泊三醇的核磁共振谱解析

利用核磁共振氢谱（¹H NMR）、碳谱（¹³C NMR）、同核位移相关谱（¹H-¹H COSY）、异核单量子相干谱（HSQC）和异核多键相关谱（HMBC）等多种核磁研究方法对抗银屑病药物卡泊三醇的¹H、¹³C信号进行完全归属.     

Porous irregular aggregates of sub-micron sized grains to reproduce cometary dust light scattering observations

The present study intends to interpret some of the characteristic features of the light scattered by cometary dust, such as phase angle and wavelength dependence of its polarization, through simulations using Ballistic Cluster-Cluster Aggregation (BCCA) or Ballistic Particle-Cluster Aggregation (BPCA) aggregates of up to 128 sub-micron sized grains (spherical and spheroidal with a possible size distribution) of various composition (silicates, organics, silicates core with organics mantle). The dependence of the linear polarization with the size parameter is shown to depend highly on the size and composition of the constitutive grains. Internal interactions induced by shape or orientation averaging of the grains may lessen this dependence, leading to results comparable to those observed on cometary dust for fluffy aggregates of grains with a size parameter in the 1.3–1.8 range. A size distribution of realistically shaped particles (aggregates of spheroids and larger spheroids) following a power law size distribution with a power index of -3, the smallest grains radius by 0.03– and the largest spheroids effective radius by , gives a very good fit to the Hale-Bopp observed phase curves. The best silicates–organics ratio ranges from about 50–75% organics and 25–50% silicates in volume considering the eventual presence of core-mantle grains.     

Channeled spectropolarimetry using a coherent white-light continuum

We carry out polarization measurements using a coherent white-light continuum as a light source for channeled spectropolarimetry. The white-light continuum, whose spectrum ranges from the UV to the IR region, is generated in Kr gas by a terawatt femtosecond laser system. The complete set of Stokes parameters from 450-700 nm are reconstructed from one spectral measurement. Also, the effectiveness of channeled spectropolarimetry using a coherent white-light continuum is experimentally demonstrated with a highly attenuating sample whose transmittance is as low as 10(-6).     

Temperature Dependence for the Relative Raman Cross Section of the Ammonia/Water Complex

The Raman spectra of aqueous ammonia solutions have been obtained between -40 and 25 degrees C. The Raman spectrum of neat water was also obtained at 25 degrees C and is characterized by two broad peaks observed at 3200 and 3400 cm(-1). The spectrum due to water is subtracted to determine the NH(3) spectrum at all temperatures. In ammonia-water solutions, the spectrum shows three features at measured displacements of 3250, 3316, and 3400 cm(-1). The feature at 3316 cm(-1) is assigned to the Q branch of the symmetric stretch. The broad, weak features at 3250 and 3400 cm(-1), previously assigned to rotational bands, are assigned to combination bands. The NH(3) combination bands are assigned by comparing with sum frequency generation (SFG) experiments, monitoring changes with temperature, and analyzing the polarization data. The rotational structure of the Q band is also discussed. As the temperature is lowered from 25 to -40 degrees C, an increase in the Raman intensity is observed for all bands. The relative Raman scattering cross section is determined from the numerically integrated area of the NH(3) Q branch at each temperature. Copyright 2001 Academic Press.     

植物叶片润湿性与光谱反射偏振度关系研究

城市绿化植物由于其滞尘能力能够有效改善城市大气环境，滞尘能力的主要影响因素是其表面特性和润湿性。植物叶片表面的润湿性和偏振反射特性都与表面粗糙度相关，因此以粗糙度为结点，探究植物表面润湿性与偏振反射光谱的关系，利用偏振反射特性对植物叶片润湿性进行快速无损监测。以长春市常见的八种绿色植物叶片为研究对象，利用Kruss DSA 100液滴形状分析系统测量其表面接触角，利用多角度光谱测量装置测量其在不同探测天顶角上的偏振反射光谱，探讨了润湿性能在400～1 000 nm波段对植物叶片不同探测天顶角上偏振度的影响。通过相关分析确定八种植物叶片的特征波段，并在特征波段对偏振度随探测天顶角的变化程度与润湿性的关系之间进行定量分析。结果表明，在特征波段(700±10) nm处，偏振度随着探测天顶角的变化程度θ₀与接触角距离100°的偏差值θ具有很强的相关性，可决系数R2为0.82，说明偏振度随探测天顶角的变化程度θ₀可以反映植物叶片的润湿性，可以利用偏振反射特性对植物叶片润湿性进行分析，研究成果将对今后城市绿化物种的选择提供可行性建议。     

温度对二氧化硫紫外吸收特性的影响

以氘灯为激发源,获得了不同温度、大气压条件下SO₂分子在285.0～320.0 nm波长区间的吸收谱。光谱图由弥散谱叠加分离谱组成,弥散谱产生于SO₂分子由基态向密集的高振转能级的吸收跃迁,分离谱主要产生于B1B₁←X1A₁跃迁,由分离谱带的间隔,获得了B1B₁激发电子态的对称拉伸和弯曲振动频率分别为：ω₁=(665±29) cm-1,ω₂=(448±17) cm-1。通过观测不同温度下的吸收谱,发现不同温度下吸收谱的结构基本相同,但吸收截面随温度升高而下降,不同吸收峰对应的吸收截面随温度呈现三次方规律变化,但比例系数不同。所以,在以吸收截面为基础的SO₂差分吸收光谱探测中,必须考虑温度变化对测量结果的影响。     

溶胶-凝胶法合成Sr3MgSi2O8:Eu3+ 红色荧光粉及其发光性能

采用溶胶-凝胶法合成了Sr_3MgSi_2O_8∶Eu~(3+)红色荧光粉,并通过X射线衍射(XRD)、傅立叶变换红外光谱(FTIR)、扫描电镜(SEM)以及荧光光谱(PL)、X射线光电子能谱(XPS)等分析方法,对不同烧结温度(950,1 050,1 150,1 250℃)热处理4 h后得到的粉体的物相、形貌及发光性能进行了表征分析。由XRD表征结果可知,制备出的样品为正交晶系的Sr_3MgSi_2O_8;由扫描电镜及比表面积的测试结果可知,经过1 150℃热处理得到的荧光粉具有最大比表面积,可达到90.213 1 m~2/g,且粒径大小均一;由红外光谱图可知:样品中含有Si—O—Si、Mg—O和Sr—O化学键;在394 nm的激发波长下,所有样品在614 nm和702 nm处出现了强发射峰,分别对应于Eu~(3+)的~5D_0→~7F_2和~5D_0→~7F_4跃迁,属红光发射。随着热处理温度的升高,样品所对应的发射峰强度呈现出先增大后减小的变化趋势,即出现温度猝灭效应;经1 150℃热处理合成的Sr_3MgSi_2O_8∶Eu~(3+)红色荧光粉的发光性能最好,即最佳的热处理温度为1 150℃。     

Polarization properties of porous anodic alumina with Y-branched Cu nanowires

Porous anodic alumina (PAA) templates with branch structure are fabricated by the two-step anodic oxidation processes, and then the Y-branched Cu nanowires are synthesized in the templates using an alternating current (AC) deposition method. We observe the morphology image of the samples by scanning electron microscopy (SEM), and measure the transmission spectrum and the polarization spectrum of the samples by the spectrophotometer. The results show that PAA films with Y-branched Cu nanowires have better transmittance in the near infrared region. An extinction ratio of 15-18 dB and an insertion loss of 0.1-0.4 dB are obtained in this region. Therefore PAA with Y-branched Cu nanowires can be used as a near-infrared micropolarizer, and this kind of micropolarizer would have a promising future in the field of photoelectricity integration.     

An inter-comparison of far-infrared line-by-line radiative transfer models

A considerable fraction (>40%) of the outgoing longwave radiation escapes from the Earth's atmosphere-surface system within a region of the spectrum known as the far-infrared (wave-numbers less than 650 cm⁻¹). Dominated by the line and continuum spectral features of the pure rotation band of water vapor, the far-infrared has a strong influence upon the radiative balance of the troposphere, and hence upon the climate of the Earth. Despite the importance of the far-infrared contribution, however, very few spectrally resolved observations have been made of the atmosphere for wave-numbers less than 650 cm⁻¹. The National Aeronautics and Space Administration (NASA), under its Instrument Incubator Program (IIP), is currently developing technology that will enable routine, space-based spectral measurements of the far-infrared. As part of NASA's IIP, the Far-Infrared Spectroscopy of the Troposphere (FIRST) project is developing an instrument that will have the capability of measuring the spectrum over the range from 100 to 1000 cm⁻¹ at a resolution of 0.6 cm⁻¹. To properly analyze the data from the FIRST instrument, accurate radiative transfer models will be required. Unlike the mid-infrared, however, no inter-comparison of codes has been performed for the far-infrared. Thus, in parallel with the development of the FIRST instrument, an investigation has been undertaken to inter-compare radiative transfer models for potential use in the analysis of far-infrared measurements. The initial phase of this investigation has focused upon the inter-comparison of six distinct line-by-line models. The results from this study have demonstrated remarkably good agreement among the models, with differences being of order 0.5%, thereby providing a high measure of confidence in our ability to accurately compute spectral radiances in the far-infrared.     

呼吸性粉尘吸收系数的光声光谱探测

针对呼吸性粉尘浓度连续、可靠、低成本的实时检测需求,实现了光谱应用技术创新,提出了一种基于光声光谱的呼吸性粉尘探测系统,低功率二极管激光器光谱中心波长为403.56 nm及相应的NO₂有效吸收截面为5.948 5×10-19 cm2·mole-1;通过频率扫描拟合得到了1.35 kHz的谐振频率。开展了光声池结构的影响分析,得到了光声池长度参数对本底噪声影响较小但对激光信号影响较大、内径参数对本底噪声存在一定影响但对本底噪声影响较小的结论。在考虑品质因数、加工条件、使用场合和待测对象属性等影响情况下,选用120 mm的长度参数和8 mm的内径参数;基于长度为60 mm、内径为25 mm的缓冲腔结构,开展了缓冲隔板对系统稳定性的影响分析,通过在缓冲腔中设置缓冲隔板,降低了本底噪声、稳定了系统,其幅值及波动由(2.83±0.11) μv稳定为(1.26±0.03) μv。分析得到了NO₂的比吸收系数为195.28 Mm-1·(mg·m-3)-1,利用NO₂气体在405 nm处的吸收对系统进行了标定,得到了拟合斜率为0.0436 8 μv/Mm-1、相关系数为0.998、池常数为300.24 Pa·cm·W-1的结论。同时在1 min平均时间下,得到了系统探测浓度下限及吸收系数为2.30 μg·m-3和0.448 Mm-1。基于标准微球的聚苯乙烯作为气溶胶发生器对象开展了呼吸性粉尘的吸收系数影响分析,进行了5μm以下不同数浓度颗粒及同一数浓度下不同粒径颗粒吸收系数的测试,结果表明：呼吸性粉尘的吸收系数和数浓度成正比,线性拟合后的斜率为10.598±0.641 96,相关系数为0.993;吸收系数曲线的方差在3～4 Mm-1间,不同粒径的颗粒对吸收系数存在着一定的影响;随着粒径增加,吸收系数随之增加。开展了环境大气中NO₂的测量,选用0.2 μm的过滤膜滤除粉尘的干扰,实验结果表明大气NO₂浓度为16.4~61.6 μg·m-3,平均浓度为41.1 μg·m-3。为了证实测量系统的准确性,与课题组自行研发的长光程差分吸收光谱系统(LP-DOAS)进行了对比测试,测试结果显示了本光声光谱系统和LP-DOAS系统测量NO₂浓度的相关性较好,线性拟合后的斜率为1.011 78±0.040 13,相关系数为0.947 81。开展了环境大气中呼吸性粉尘的测量,选用5 μm过滤片过滤环境大气,通过“NO₂+5 μm粉尘”和“NO₂+0.2 μm粉尘”两路测量对象的差分测量,得到了呼吸性粉尘的变化趋势,可以满足自然悬浮状态下的呼吸性粉尘吸收系数实时测量。     

以NMR手段为主进行结构分析一则

某固体化合物极易溶于水但未能制得其单晶.为测定其结构可充分发挥NMR的特长.利用NMR的多种技术,包括常规¹H谱、¹³C谱,一维、二维谱,确定脂肪链X-CH₂-CH₂-CH₂-Y与芳香基团Y的存在;利用¹³C-¹H长程耦合相关谱(用反相探头)可确定脂肪链与芳香基团间的联接关系,于是立即可确定该物的有机主体骨架应为X-CH₂-CH₂-CH₂-Y.从IR谱得知该物除存在C-N基团外,还存在-SO₃-基团等.从元素成分分析和M8谱,包括FAB-MS谱等分析结果,最终推断该固体化合物为丙磺酸吡啶内鎓盐,它的分子结构应为:      

基于连续谱的氩气纳秒脉冲放电中电子温度的研究

采用了一种针对针的放电结构,将其放置在一个高纯氩气的密闭腔室中,通过施加正极性的过电压产生可重复的大气压纳秒脉冲放电,并提出建立大气压放电的连续辐射模型来诊断氩气纳秒脉冲放电中的电子温度。实验利用电压和电流探头分别获取放电过程中的电压和电流波形图,其放电脉宽约为20 ns。通过消色差透镜、单色仪和ICCD等光学系统的组合来测量放电正柱区在不同时刻（0<t<20 ns）的时间分辨发射光谱。结果表明,放电中连续谱的强度随时间先增加（0<t<10 ns）后减小（10 ns<t<20 ns）,但是氩原子的谱线强度则随时间的增加而一直增大。研究表明连续谱强度与电子密度成正相关,因而电子密度随着时间也是先增加而后减小,这与放电电流的变化规律是完全一致的。根据连续谱模型拟合得到放电过程中（0<t<10 ns）的电子温度为(1.4±0.2) eV。随着驱动电压的下降（10 ns<t<20 ns）,电子温度逐步减小至0.9 eV。在0<t<10 ns中,激发态氩原子主要是由电子碰撞激发产生的,因而谱线强度随着电子密度的增加而增大。然后,随着电子温度的减小,Ar+₂复合反应速率激增,导致电子与离子的复合过程主导产生激发态氩原子,即谱线强度继续增大。通过加入0.5%的水蒸气以获取OH的振转光谱。实验发现,OH(A)的产生机制使其偏离玻尔兹曼平衡分布,本文采用了双温的OH(A-X)光谱模型来考察气体温度。在放电过程中,气体温度保持不变,大约为400 K。此外,水蒸气的加入使得短波长的连续谱发生显著增强。光谱分析认为H₂O在放电中能够解离产生H₂,继而与氩原子的亚稳态发生能量转移生成激发态H₂(a3Σ+_g)。H₂(a3Σ+_g)将会自发辐射跃迁到排除态H₂(b3Σ+_u),同时发射短波长的连续谱。由于短波长的连续谱对电子温度（T_e>1 eV）的响应较为灵敏,所以载气中少量的水蒸气将会对连续谱诊断电子温度带来较大的影响。     

锶原子互组跃迁谱的实验研究

观测了热原子束中锶原子(5s²)¹S₀—(5s5p)³P₁ 互组跃迁的荧光谱和饱和荧光谱, 实验测定了不同的参量下 ⁸⁸Sr(5s²)¹S₀—(5s5p)³P₁ 互组跃迁荧光谱. 研究结果表明: 不同的实验参数(温度、激光光强、激光扫描频率)对谱线有较大的影响, (5s5p)³P₁态的长寿命特点导致谱线强度随激光扫描频率呈倒数关系变化; 由于Doppler增宽, 渡越增宽等因素的影响, 谱线线宽远大于其自然线宽且随激光扫描频率呈正比关系变化.     

超短脉冲激光烧蚀石墨产生的喷射物的时间分辨发射光谱研究

本文使用不同激光能流(18 J/cm²–115 J/cm²)和脉冲宽度(50 fs–4 ps)的超短脉冲激光在真空中(4×10^-4 Pa)烧蚀高定向热解石墨. 通过测量烧蚀喷射物的时间分辨发射光谱研究喷射物的超快时间演化. 在喷射物发射光谱中, 观察到了C₂基团的天鹅带光谱系统, 416 nm附近C₁₅基团的由电子能级¹Σ_u⁺ 和¹Σ_g⁺之间的振动跃迁产生的光谱峰以及连续谱. 50 fs, 115 J/cm²的脉冲激光烧蚀产生的喷射物的连续谱的强度衰减分为快速下降和慢速下降两个阶段(以20 ns时间延迟为分界). 这表明连续谱是由两种不同的组分贡献的. 快速下降阶段, 连续谱主要由碳等离子体通过韧致辐射产生; 慢速下降阶段, 连续谱主要由烧蚀后期产生的大颗粒碳簇的热辐射贡献. 实验结果还揭示了激光能流的提高, 会明显增加喷射物中碳等离子体和激发态C₂的含量, 但对质量稍大的C₁₅的影响较小; 此外, 50 fs脉冲激光烧蚀产生的连续谱的存在时间会随着激光能流的减小而增大, 这说明低能流更有利于在烧蚀后期产生碳簇. 脉宽主要影响喷射物连续谱的时间演化. 4 ps脉冲激光烧蚀产生的连续谱的整个时间演化过程明显慢于50 fs脉冲产生的连续谱.     

Polarisation properties of comet NEAT C/2001 Q4

Comets exhibit high (up to 25 %) amount of optical polarization when they are observed through ground based or space telescopes. These polarizations are caused due to the scattering of cometary dust. The observed linear polarisation of comets is generally a function of the wavelength of incident light (λ), the scattering angle (θ), the geometrical shape and size of the particle and the composition of dust particles in terms of the complex values of the refractive index. The scattering properties of cometary dust will help to know the nature of cometary dust. In the present work, the observed linear polarization data of Comet NEAT are studied through simulations using Ballistic Particle-Cluster Aggregate (BPCA) and Ballistic Cluster-Cluster Aggregate (BCCA). Using Superposition T-matrix code, the best-fitting values of complex refractive indices are calculated which can well fit the observed polarization data of Comet NEAT C/2001 Q4. The best fitting values of complex refractive indices coming out from the present analysis correspond to mixture of both silicates and organics.