991 resultados para Ólafsdóttur, Þórunn, 1764-1786.


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采用单电子模型研究了圆偏振飞秒脉冲激光作用下电子振荡导致的谐波辐射频谱的特性。研究发现随着激光强度的增加,电子在激光场中运动的相对论效应可以导致谐波辐射,并且发现谐波辐射频谱随着激光强度的增加发生了展宽和红移。电子与强激光脉冲相互作用,电子除了在激光场的作用下做横向振荡运动之外,激光脉冲的纵向有质动力对电子还有推动作用,这是产生谐波频谱红移的原因,而谐波辐射频谱展宽是由电子纵向速度的变化引起的。分析激光场中电子在不同方向的辐射频谱表明:随着谐波阶数的升高,红移在有规律地变大;在θ=3π/4方向上电子频谱的

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Ephemera danica Müller, 1764 (Ephemeroptera) is one of the largest mayflies found in the British Isles with some females reaching over 30 mm. It is a common and widespread species found in rivers, lakes and streams throughout Europe and is particularly abundant in many of the lowland rivers of the British Isles. The larvae are burrowers – mainly found where silt accumulates below macrophytes. This article gives a general overview of research work on the factors affecting the life cycle of Ephemera danica over a seven year period (1995–2002) on two rivers, the River Test at Leckford in Hampshire and the North Wey at Tilford in Surrey.

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Modulated UV light is used to increase the sensitivity of the two-centre holographic recording. Inherent mechanisms of nonvolatile holographic recording in oxidized and reduced crystals are numerically analysed based on solving the two-centre material equations modilied for UV-Iight recording. Experiments verification is performed with an oxidized crystal and a reduced crystal, and the role of UV intensity on the sensitivity is presented.

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The degradation of image quality caused by aberrations of projection optics in lithographic tools is a serious problem in optical lithography. We propose what we believe to be a novel technique for measuring aberrations of projection optics based on two-beam interference theory. By utilizing the partial coherent imaging theory, a novel model that accurately characterizes the relative image displacement of a fine grating pattern to a large pattern induced by aberrations is derived. Both even and odd aberrations are extracted independently from the relative image displacements of the printed patterns by two-beam interference imaging of the zeroth and positive first orders. The simulation results show that by using this technique we can measure the aberrations present in the lithographic tool with higher accuracy. (c) 2006 Optical Society of America.

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本研究在野外调查的基础上,采用随机扩增多态DNA (RAPD)分析和形态学方法,研究了我国三种珍稀濒危兰科植物硬叶兜兰(Paphiopedilummicranthum)、麻栗坡兜兰(P. malipoense)和独花兰(Changnienia amoena)的遗传多样性与群体遗传结构,主要结果如下: 1.采用1 2个引物对分布于我国云贵地区的4个硬叶兜兰群体共161个体进行RAPD扩增和分析,得出物种水平的多态条带百分率(PPB)为71.6%,Nci的基因多样度(h)为0.217,Shannon多样性指数(1)为0.3301;4个群体的平均多样性水平为PPB=45.2%,h=0.1457,1= 0.2204:低于远交兰花的平均水平。分子方差分析(AMOVA)表明,在总遗传变异中,群体间遗传变异占20.31%.群体内占79.69%;POPGENE给出的基因分化系数 (Gst)为0.2958;遗传分化略高于远交物种的平均水平。空间自相关分析表明,所检测的两个群体中存在明显的空间结构,基因型在群体中以不同的小斑块存在。遗传距离和空间距离不存在相关关系。 2.用于麻栗坡兜兰的RAPD引物同上,但取样范围只有贵州的2个群体共10个个体。就所研究的个体柬看,麻栗坡兜兰的遗传多样性明显低于远交兰花物种的平均水平。物种水平上,多态条带百分率(PPB)为49.5%。Nei的基因多样度(h)为0. 1174, Shannon多样性指数(I)为0.1764:在群体水平上,上述三个指标的平均值则分别为12. 75%、0.0486和0.0712,均大大低于硬叶兜兰。然而,尽管作了种种努力,麻栗坡兜兰的取样个体数量仍很少,因此所得结果可能会有误差。 3.用16个引物对分布于河南、湖北、湖南、江西4个省11个独花兰群体共216个体进行了RA PD扩增和分析,独花兰在物种水平PPB=80. 7%,h=0.197.1=0. 3116;在群体水平,上述三个指标的平均值则分别为40. 9%、0.1247和0. 1902,均低于远交兰花的平均水平。AMOVA分析表明,11个独花兰群体间的遗传变异占43.48%,群体内的占56.52%:在神农架和新宁地区内部,群体间的遗传变异分别占13.68%和49.3g%(AMOVA)。POPGENE给出的11个群体的基因分化系数(Gst)为0.3580.神农架和新宁地区内的Gst,值分别为0.1194和0.2597。可见,群体间的遗传分化明显高于远交物种的平均水平。空间自相关分析表明,独花兰的遗传变异在群体内不存在明显的空间结构。群体之间的遗传距离和空间距离不存在相关关系。 4.对独花兰7个群体形态性状的分析发现,12个形态性状在群体内均有较高的变异性,cv值变动于0.022-0.30O。庐山群体(LS)在所有性状上的平均值均为最高。营养性状和花部性状的变异性基本一致。除花葶长和花距直径与某些花部性状之间没有显著的相关关系外,各性状之间均有显著的相关性。对XN4群体的统计没有发现假磷茎数目与其他性状之间存在显著相关性。 根据以上对硬叶兜兰、麻粟坡兜兰和独花兰遗传多样性和群体遗传结构韵研究,结合其他方面的资料;对三种兰花的濒危机制进行了初步的分析。首先,人为采挖和破坏是导致这些兰花物种濒危的直接原因,尤其是麻栗坡兜兰。其次, 适宜兰花生存的生境正在只益萎缩、退化和片段化。这两方面因素的共同作用导致上述兰花群体的数目和规模日益下降,由此引发的遗传多样性降低和遗传结构的改变进一步加剧其濒危状况。对于独花兰而言,较低的繁殖能力又使其生存状态雪上加霜。针对三个物种不同的繁殖特性和遗传学状况,提出如下保护措施。(1)硬叶兜兰由于繁殖能力较强、现存个体尚多,遗传多样性损失不甚严重,因此以保护其所在的生境为基础、实施原位保护,是比较合适的保护策略。(2)麻粟坡兜兰目前受破坏程度非常严重;所剩个体很少,遗传多样性较低,已经很难进行有效的原位保护。因此;应利用迁地保护手段抢救目前尚存的个体。(3)独花兰的繁殖能力较弱,因此在保护生境和严禁采摘的基础上,可采用人工授粉等方式,提高结实率、增加繁殖效率,促使其复壮:在进行迁地保护时,则应注意不同群体间存在较大遗传变异而群体内多样性较低这一现实。