Lemmens Alexander K, Mackie Cameron J, Candian Alessandra, Lee Timothy M J, Tielens Alexander G G M, Rijs Anouk M, Buma Wybren Jan
Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, 1098 XH Amsterdam, The Netherlands.
Radboud University, Institute for Molecules and Materials, FELIX Laboratory, 6525 ED Nijmegen, The Netherlands.
Faraday Discuss. 2023 Sep 20;245(0):380-390. doi: 10.1039/d2fd00180b.
The intensity ratio of the 11.2/3.3 μm emission bands is considered to be a reliable tracer of the size distribution of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). This paper describes the validation of the calculated intrinsic infrared (IR) spectra of PAHs that underlie the interpretation of the observed ratio. The comparison of harmonic calculations from the NASA Ames PAH IR spectroscopic database to gas-phase experimental absorption IR spectra reveals a consistent underestimation of the 11.2/3.3 μm intensity ratio by 34%. IR spectra based on higher level anharmonic calculations, on the other hand, are in very good agreement with the experiments. While there are indications that the 11.2/3.3 μm ratio increases systematically for PAHs in the relevant size range when using a larger basis set, it is unfortunately not yet possible to reliably calculate anharmonic spectra for large PAHs. Based on these considerations, we have adjusted the intrinsic ratio of these modes and incorporated this in an interstellar PAH emission model. This corrected model implies that typical PAH sizes in reflection nebulae such as NGC 7023 - previously inferred to be in the range of 50 to 70 carbon atoms per PAH are actually in the range of 40 to 55 carbon atoms. The higher limit of this range is close to the size of the C fullerene (also detected in reflection nebulae), which would be in line with the hypothesis that, under appropriate conditions, large PAHs are converted into the more stable fullerenes in the ISM.
11.2/3.3μm发射带的强度比被认为是星际介质（ISM）中多环芳烃（PAH）尺寸分布的可靠示踪剂。本文描述了对PAH计算固有红外（IR）光谱的验证，该光谱是观测比值解释的基础。将美国国家航空航天局艾姆斯研究中心PAH红外光谱数据库的谐波计算结果与气相实验吸收红外光谱进行比较，结果表明11.2/3.3μm强度比始终被低估34%。另一方面，基于更高水平非谐计算的红外光谱与实验结果非常吻合。虽然有迹象表明，在使用更大基组时，相关尺寸范围内的PAH的11.2/3.3μm比值会系统性增加，但不幸的是，目前还无法可靠地计算大型PAH的非谐光谱。基于这些考虑，我们调整了这些模式的固有比值，并将其纳入星际PAH发射模型。这个修正后的模型表明，反射星云（如NGC 7023）中典型的PAH尺寸——之前推断为每个PAH含有50至70个碳原子——实际上在40至55个碳原子的范围内。这个范围的上限接近C富勒烯的尺寸（在反射星云中也有检测到），这与在适当条件下，大型PAH在星际介质中转化为更稳定的富勒烯这一假设相符。
