5.25与5.7 μm天文多环芳香烃发射特征
The 5.25 & 5.7 $μ$m Astronomical Polycyclic Aromatic Hydrocarbon Emission Features
AI总结：
本研究结合实验室光谱与量子化学计算，分析ISO SWS中5.25和5.7 μm弱PAH发射特征，揭示其组合、差频与倍频带来源，并指示大型、紧凑规则的中性PAH族群。
摘要：
天文中红外光谱在5.25和5.7 μm（1905和1754 cm⁻¹）处显示出两个较弱的PAH特征；迄今为止，这些特征很少受到研究，但它们包含关于天文PAH族群的信息，可补充主要发射带所携带的信息。在此，我们报告一项结合实验室与理论分析的研究，考察在该区域产生特征的PAH光谱学基频振动，并利用这些结果分析天文光谱。本研究考虑了15个显示这些PAH特征的天体的ISO SWS光谱，其中4个在5至6 μm之间具有足够的信噪比（S/N），可进行深入分析。全部4个天文光谱均显示出相似的峰值位置和轮廓。5.25 μm特征呈尖峰且不对称，而5.7 μm特征更宽、更平。对实验室光谱和量子化学计算的详细分析表明，天文上的5.25和5.7 μm谱带是组合带、差频带和倍频带的混合，主要涉及CH伸缩振动以及CH面内弯曲和面外弯曲基频振动。实验和计算光谱显示，在PAHs上可能存在的所有氢邻接类别中，孤立氢（solo）和成对氢（duo）始终在观测位置产生显著谱带，而四连氢（quartet）则不会。总体而言，这项研究支持天文PAHs为具有紧凑、规则结构的大型分子这一图景。根据其主要与强CH面外弯曲模式的耦合，人们可以推测5.25和5.7 μm谱带追踪中性PAH族群。然而，理论表明，电荷在这些天文谱带中的作用可能也很重要。
英文摘要：
Astronomical mid-IR spectra show two minor PAH features at 5.25 and 5.7 $μ$m (1905 and 1754 cm$^{\rm - 1}$) that hitherto have been little studied, but contain information about the astronomical PAH population that complements that of the major emission bands. Here we report a study involving both laboratory and theoretical analysis of the fundamentals of PAH spectroscopy that produce features in this region and use these to analyze the astronomical spectra. The ISO SWS spectra of fifteen objects showing these PAH features were considered for this study, of which four have sufficient S/N between 5 and 6 $μ$m to allow for an in-depth analysis. All four astronomical spectra show similar peak positions and profiles. The 5.25 $μ$m feature is peaked and asymmetric, while the 5.7 $μ$m feature is broader and flatter. Detailed analysis of the laboratory spectra and quantum chemical calculations show that the astronomical 5.25 and 5.7 $μ$m bands are a blend of combination, difference and overtone bands primarily involving CH stretching and CH in-plane and CH out-of-plane bending fundamental vibrations. The experimental and computational spectra show that, of all the hydrogen adjacency classes possible on PAHs, solo and duo hydrogens consistently produce prominent bands at the observed positions whereas quartet hydrogens do not. In all, this a study supports the picture that astronomical PAHs are large with compact, regular structures. From the coupling with primarily strong CH out-of-plane bending modes one might surmise that the 5.25 and 5.7 $μ$m bands track the neutral PAH population. However, theory suggests the role of charge in these astronomical bands might also be important.
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