Infrared Spectroscopy of Diamondoid Molecules: New Insights into the Presence of Nanodiamonds in the Interstellar Medium
Infrared Spectroscopy of Diamondoid Molecules: New Insights into the Presence of Nanodiamonds in the Interstellar Medium
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DOI：
10.1086/516731
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发表时间：
2007-06
期刊：
影响因子：
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通讯作者：
O. Pirali;M. Vervloet;Jeremy E. Dahl;Robert M. K. Carlson;A. Tielens;J. Oomens
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文献类型：
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作者：
O. Pirali;M. Vervloet;Jeremy E. Dahl;Robert M. K. Carlson;A. Tielens;J. Oomens
虽然它们的波段形状相对不同，但HD 97048和Elias 1的发射光谱以及各种致密云的吸收光谱中3.4-3.5 μm左右的红外特征都归因于金刚石分子/颗粒。这项任务主要基于氢化金刚石薄膜和已知平均尺寸的金刚石纳米晶体的红外光谱。在这里，我们对最近报道的单个金刚石分子的固态2.5-12.5 μm光谱进行了天体物理学意义的分析，这些分子的大小可达六正烷（C26H30）。这些光谱提供的第一个实验测量这类分子的红外频率。此外，还报道了三种最小的类金刚石的实验室气相红外发射光谱，以及一些较大的类金刚石的理论光谱。目前的数据集使我们能够将光谱特征与分子大小和结构联系起来。发现四面体菱形体的光谱与低对称种的光谱有质的不同，这可能解释了天体物理发射和吸收光谱之间的差异。有趣的是，在这些小分子类金刚石的光谱中可以清晰地观察到3.53 μm的波段，而之前对纳米金刚石颗粒的研究发现，这个波段只存在于大于≈50 nm的纳米金刚石颗粒中。我们的研究结果支持了HD 97048和Elias 1中3.43和3.53 μm的发射特征是由几纳米大小的金刚石分子引起的，以及较小的金刚石分子对3.47 μm星际吸收带的贡献。
Although they are relatively different in band shape, infrared features around 3.4-3.5 μm in the emission spectra of HD 97048 and Elias 1 and in the absorption spectra of various dense clouds have both been attributed to diamondoid molecules/particles. This assignment is based mainly on infrared spectra of hydrogenated diamond thin films and of diamond nanocrystals of known average size. Here we present an analysis of the astrophysical implications of recently reported solid-state 2.5-12.5 μm spectra of individual diamondoid molecules, up to the size of hexamantane (C26H30). These spectra provide the first experimental measurements of the infrared frequencies of this class of molecules. In addition, laboratory gas-phase infrared emission spectra of the three smallest members of the diamondoid family are reported, as well as theoretical spectra for some larger species. The present data set allows us to relate spectral signatures to the molecular size and structure. The spectra of tetrahedral diamondoids are found to be qualitatively different from those of lower symmetry species, which possibly explains the differences between the astrophysical emission and absorption spectra. Interestingly, the 3.53 μm band is clearly observed in the spectra of these small molecular diamondoids, whereas previous studies on nanodiamond particles found this band only for species larger than ≈50 nm. Our results support the assignment of the 3.43 and 3.53 μm emission features in HD 97048 and Elias 1 to diamondoids of a few nanometers in size as well as the suggestion that smaller diamondoid molecules contribute to the 3.47 μm interstellar absorption band.