Snow T P, Le Page V, Keheyan Y, Bierbaum V M
Center for Astrophysics and Space Astronomy, University of Colorado, Boulder 80309, USA.
Nature. 1998 Jan 15;391(6664):259-60. doi: 10.1038/34602.
Diffuse interstellar bands (DIBs) are mysterious absorption lines in the optical spectra of stars, and have been known for 75 years. Although it is widely believed that they arise from gas-phase organic molecules (rather than from dust grains) in the interstellar medium, no consensus has been reached regarding their precise cause. The realization that many emission features in astronomical infrared spectra probably arise from polycyclic aromatic hydrocarbons (PAHs), which may themselves be very abundant in the interstellar medium, has led to the suggestion that ionized PAHs might be the source of the DIBs. Laboratory investigations have revealed that small, positively charged PAHs in matrices have absorption features that bear some resemblance to DIBs, but no clear identification of any DIB with any specific PAH cation has yet been made. Here we report a laboratory study of the chemical reactivity of PAH cations (C6H6+, C10H8+ and C16H10+) in the gas phase. We find that these PAH cations are very reactive, and are therefore unlikely to survive in high abundances in the interstellar medium. Rather, such molecules will react rapidly with hydrogen, and we therefore suggest that the resulting protonated PAH cations (and species derived from them) should become the focus of future searches for a correspondence between molecular absorption features and the DIBs.
弥漫星际带（DIBs）是恒星光谱中的神秘吸收线，已被发现75年了。尽管人们普遍认为它们起源于星际介质中的气相有机分子（而非尘埃颗粒），但其确切成因尚未达成共识。意识到天文红外光谱中的许多发射特征可能源于多环芳烃（PAHs），而多环芳烃本身在星际介质中可能非常丰富，这引发了一种观点，即电离的多环芳烃可能是弥漫星际带的来源。实验室研究表明，基质中的小的带正电荷的多环芳烃具有与弥漫星际带有些相似的吸收特征，但尚未明确将任何弥漫星际带与任何特定的多环芳烃阳离子进行识别。在此我们报告一项关于气相中多环芳烃阳离子（C6H6 +、C10H8 +和C16H10 +）化学反应性的实验室研究。我们发现这些多环芳烃阳离子非常活泼，因此不太可能在星际介质中大量存在。相反，这类分子会与氢迅速反应，因此我们认为由此产生的质子化多环芳烃阳离子（以及由它们衍生的物种）应成为未来寻找分子吸收特征与弥漫星际带之间对应关系的重点。
