星际芳香红外带：萘的最低三重态的红外吸收光谱
Interstellar Aromatic Infrared Bands: IR absorption spectroscopy of the lowest triplet state of naphthalene
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中文总结 AI 辅助
本文首次测量萘的最低三重态红外吸收光谱，发现其与单重态基态光谱差异显著，该成果为星际介质中三重态PAHs的探测及更准确红外发射模型的开发提供了基础。
中文摘要 AI 辅助
芳香红外带（AIBs）是广泛存在于各类星际天体中的发射特征，通常归因于多环芳烃（PAHs）吸收紫外光子并发生内转换后，处于高激发基态振动能级的辐射冷却过程。不过对于中性PAHs而言，系间窜越至三重态，随后在最低激发三重态内发生振动能级的辐射冷却，被认为是一种竞争弛豫通道。该状态的振动频率、跃迁偶极矩和内能均存在差异，因此会产生不同的发射光谱。尽管中性PAHs的三重态具有重要意义，但由于缺乏直接的光谱探测手段，其研究仍处于空白状态。本文中，我们首次报道了萘的最低三重态的红外吸收光谱测量结果。该光谱与单重态电子基态的振动光谱存在显著差异，其特征可用于在天文光谱中识别处于三重态的PAHs。我们还表明，实验光谱可通过非谐频率计算得到精确复现。这些发现为在星际介质中探测三重态PAHs奠定了基础，并为开发更准确的红外发射模型提供了依据。
英文摘要
The aromatic infrared bands (AIBs), emission features observed ubiquitously across a wide variety of interstellar objects, are commonly attributed to radiative cooling of highly-excited ground-state vibrational levels of polycyclic aromatic hydrocarbons (PAHs) that are populated after UV absorption and internal conversion. However, for neutral PAHs intersystem crossing to the triplet manifold, followed by radiative cooling of vibrational levels within the lowest excited triplet state is expected to be a competing relaxation channel. This state differs in vibrational frequencies, transition moments and internal energy, and therefore gives rise to different emission spectra. Despite their significance, neutral PAHs in their triplet states have remained largely unexplored, primarily because of the lack of direct spectroscopic access. Here, we report the first measurement of the infrared absorption spectrum of the lowest triplet state of naphthalene. The spectrum is markedly different from the vibrational spectrum of the singlet electronic ground state, displaying features that can help identifying PAHs in the triplet states in astronomical spectra. We also show that experimental spectra are closely reproduced by anharmonic frequency calculations. These findings serve as a stepping stone for detecting triplet-state PAHs in the interstellar medium and provide a basis for developing more accurate infrared emission models.
