Dinu Dennis F, Bartl Pit, Quoika Patrick K, Podewitz Maren, Liedl Klaus R, Grothe Hinrich, Loerting Thomas
Institute of General, Inorganic and Theoretical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria.
Institute of Physical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria.
J Phys Chem A. 2022 May 19;126(19):2966-2975. doi: 10.1021/acs.jpca.2c00857. Epub 2022 May 9.
We performed matrix-isolation infrared (MI-IR) spectroscopy of carbon dioxide monomers, CO, and dimers, (CO), trapped in neon and in air. On the basis of vibration configuration interaction (VCI) calculations accounting for mode coupling and anharmonicity, we identify additional infrared-active bands in the MI-IR spectra due to the (CO) dimer. These bands are satellite bands next to the established CO monomer bands, which appear in the infrared window of Earth's atmosphere at around 4 and 15 μm. In a systematic carbon dioxide mixing ratio study using neon matrixes, we observe a significant fraction of the dimer at mixing ratios above 300 ppm, with a steep increase up to 1000 ppm. In neon matrix, the dimer increases the IR absorbance by about 15% at 400 ppm compared to the monomer absorbance alone. This suggests a high fraction of the (CO) dimer in our matrix experiments. In atmospheric conditions, such increased absorbance would significantly amplify radiative forcings and, thus, the greenhouse warming. To enable a comparison of our laboratory experiment with various atmospheric conditions (Earth, Mars, Venus), we compute the thermodynamics of the dimerization accordingly. The dimerization is favored at low temperatures and/or high carbon dioxide partial pressures. Thus, we argue that matrix isolation does not trap the gas composition "as is". Instead, the gas is precooled to 40 K, where CO dimerizes before being trapped in the matrix, already at very low carbon dioxide partial pressures. In the context of planetary atmospheres, our results improve understanding of the greenhouse effect for planets of rather thick CO atmospheres such as Venus, where a significant fraction of the (CO) dimer can be expected. There, the necessity of including the mid-IR absorption by stable (CO) dimers in databases used for modeling radiative forcing, such as HITRAN, arises.
我们对捕获在氖气和空气中的二氧化碳单体（CO）和二聚体（(CO)₂）进行了基质隔离红外（MI-IR）光谱研究。基于考虑模式耦合和非谐性的振动组态相互作用（VCI）计算，我们在MI-IR光谱中识别出了由于(CO)₂二聚体产生的额外红外活性带。这些带是紧邻已确定的CO单体带的卫星带，它们出现在地球大气红外窗口中约4和15μm处。在使用氖基质的系统二氧化碳混合比研究中，我们观察到在混合比高于300ppm时二聚体占很大比例，在高达1000ppm时急剧增加。在氖基质中，与仅单体吸收相比，二聚体在400ppm时使红外吸收率增加约15%。这表明在我们的基质实验中二聚体(CO)₂占比很高。在大气条件下，这种增加的吸收率将显著放大辐射强迫，进而增强温室效应。为了能将我们的实验室实验与各种大气条件（地球、火星、金星）进行比较，我们相应地计算了二聚化的热力学。二聚化在低温和/或高二氧化碳分压下更有利。因此，我们认为基质隔离并非原样捕获气体成分。相反，气体被预冷至40K，在那里CO在被捕获到基质中之前就已经二聚化，即使在非常低的二氧化碳分压下也是如此。在行星大气的背景下，我们的结果有助于更好地理解对于像金星这样具有相当浓厚CO大气层的行星的温室效应，在金星上预计会有很大比例的(CO)₂二聚体。在那里，在用于模拟辐射强迫的数据库（如HITRAN）中纳入稳定(CO)₂二聚体的中红外吸收就变得很有必要。
