星际水冰模型中一氧化碳的IR光谱指纹
IR Spectral Fingerprint of Carbon Monoxide in Interstellar Water Ice Models
摘要：
一氧化碳（CO）是星际介质中第二丰富的气相分子。在密集的分子云中，它也以固相存在，作为覆盖尘埃颗粒的混合水为主的冰的成分。其在固相中的存在是从其红外（IR）信号推断的。在固体CO/水混合样品的实验观测中，其IR频率分裂成两个分量，产生蓝移和红移带。然而，在天文观测中，前者从未被观测到。已经进行了几次尝试来解释这种特殊的行为，但问题仍然悬而未决。在这项工作中，我们采用了纯量子力学模拟，以阐明这个问题。采用不同的周期性模型模拟了CO/H $_2$ O冰体系，如CO在水冰表面的单重吸附和多重吸附、CO在水笼中的捕获以及适当的CO：H $_2$ O混合冰。我们还模拟了纯固体CO。我们的数据的详细分析揭示了如何四极字符的CO和分散力与水冰确定的CO/H $_2$ O冰相互作用的能量，以及CO光谱行为。我们的数据表明，蓝移峰可以分配给CO相互作用{ \it via}的C原子与悬挂的H原子的水冰，而红移的一个实际上可以是CO参与不同的相互作用与水矩阵的结果。我们还提供了一个可能的解释，天文光谱中的蓝移峰的缺乏。我们的目的不是提供一个完整的帐户的各种星际冰，而是要阐明的敏感性的CO光谱特征不同的水冰环境。
英文摘要：
Carbon monoxide (CO) is the second most abundant molecule in the gas-phase of the interstellar medium. In dense molecular clouds, it is also present in the solid-phase as a constituent of the mixed water-dominated ices covering dust grains. Its presence in the solid-phase is inferred from its infrared (IR) signals. In experimental observations of solid CO/water mixed samples, its IR frequency splits into two components, giving rise to a blue- and a redshifted band. However, in astronomical observations, the former has never been observed. Several attempts have been carried out to explain this peculiar behaviour, but the question still remains open. In this work, we resorted to pure quantum mechanical simulations in order to shed some light on this problem. We adopted different periodic models simulating the CO/H$_2$O ice system, such as single and multiple CO adsorption on water ice surfaces, CO entrapped into water cages and proper CO:H$_2$O mixed ices. We also simulated pure solid CO. The detailed analysis of our data revealed how the quadrupolar character of CO and the dispersive forces with water ice determine the energetic of the CO/H$_2$O ice interaction, as well as the CO spectroscopic behaviour. Our data suggest that the blueshifted peak can be assigned to CO interacting {\it via} the C atom with dangling H atoms of the water ice, while the redshifted one can actually be the result of CO involved in different reciprocal interactions with the water matrix. We also provide a possible explanation for the lack of the blueshifted peak in astronomical spectra. Our aim is not to provide a full account of the various interstellar ices, but rather to elucidate the sensitivity of the CO spectral features to different water ice environments.
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