GRAPHITE REVISITED
GRAPHITE REVISITED
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DOI：
10.3847/0004-637x/831/1/109
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发表时间：
2016-08
期刊：
影响因子：
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通讯作者：
B. Draine
中科院分区：
文献类型：
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作者：
B. Draine
实验室测量用于约束石墨的介电张量，从微波到 X 射线频率。即使在 X 射线能量下，介电张量也具有很强的各向异性。采用离散偶极近似来精确计算单晶石墨球和球体的吸收和散射。对于随机取向的单晶粒，计算吸收和散射截面的所谓近似在极限内是精确的，即使在不小的情况下，在光学和紫外光中也能提供优于~10%的精度，但在红外波长下变得越来越不准确，误差高达~40%。对于乱层石墨颗粒，布鲁格曼和麦克斯韦加内特处理在光学和紫外中产生相似的横截面，但在红外中出现分歧，预测的横截面在远红外中相差一个数量级以上。有人认为麦克斯韦加内特的估计可能更现实，因此受到推荐。詹姆斯·韦伯太空望远镜上的 MIRI 摄谱仪可以在吸收过程中观察到 11.5 μm 附近石墨的面外晶格共振。排列整齐的石墨颗粒如果存在于星际介质中，可以在碳 K 边缘附近产生偏振 X 射线吸收和偏振 X 射线散射。
Laboratory measurements are used to constrain the dielectric tensor for graphite, from microwave to X-ray frequencies. The dielectric tensor is strongly anisotropic even at X-ray energies. The discrete dipole approximation is employed for accurate calculations of absorption and scattering by single-crystal graphite spheres and spheroids. For randomly oriented single-crystal grains, the so-called approximation for calculating absorption and scattering cross sections is exact in the limit and provides better than ∼10% accuracy in the optical and UV even when is not small, but becomes increasingly inaccurate at infrared wavelengths, with errors as large as ∼40% at . For turbostratic graphite grains, the Bruggeman and Maxwell Garnett treatments yield similar cross sections in the optical and ultraviolet, but diverge in the infrared, with predicted cross sections differing by over an order of magnitude in the far-infrared. It is argued that the Maxwell Garnett estimate is likely to be more realistic, and is recommended. The out-of-plane lattice resonance of graphite near 11.5 μm may be observable in absorption with the MIRI spectrograph on James Webb Space Telescope. Aligned graphite grains, if present in the interstellar medium, could produce polarized X-ray absorption and polarized X-ray scattering near the carbon K edge.