Carbon dioxide photolysis from 150 to 210 nm: Singlet and triplet channel dynamics, UV-spectrum, and isotope effects
Carbon dioxide photolysis from 150 to 210 nm: Singlet and triplet channel dynamics, UV-spectrum, and isotope effects
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DOI：
10.1073/pnas.1213083110
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发表时间：
2013-06
期刊：
影响因子：
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通讯作者：
J. Schmidt;M. Johnson;R. Schinke
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文献类型：
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作者：
J. Schmidt;M. Johnson;R. Schinke
我们提出了 150 至 210 nm 波长范围内二氧化碳 (CO2) 光解过程的第一原理研究，重点是低于一氧化碳 + 单线态通道阈值（~167 nm）的光解作用。计算以 ~0.5 nm 的分辨率重现实验吸收截面，而无需缩放强度。所观察到的 150 至 210 nm 范围内的结构是由深井支持的弯曲运动激发引起的，弯曲几何形状位于 和势能表面。低于单重态通道阈值的预解离通过与附近的排斥三重态的自旋轨道耦合发生。单线态通道以及 157 nm 激发的三线态通道中的一氧化碳振动和旋转状态分布令人满意地再现了实验数据。计算了各个 CO2 同位素体（12C16O2、12C17O16O、12C18O16O、13C16O2 和 13C18O16O）的横截面，表明强烈的同位素分馏将作为波长的函数发生。这些计算提供了对 CO2 光吸收和解离动力学的准确、详细的了解，并极大地扩展了横截面温度依赖性的知识，以涵盖 0 到 400 K 的范围，这对于计算行星大气中恒星光的传播非常有用。该模型还与质量无关同位素分馏实验室实验的解释相关。最后，该模型表明，在一系列汞灯实验中观察到的与质量无关的分馏并不是超精细相互作用的结果，使得含有 CO2 的 17O 的预解离更加有效。
We present a first principles study of the carbon dioxide (CO2) photodissociation process in the 150- to 210-nm wavelength range, with emphasis on photolysis below the carbon monoxide + singlet channel threshold at ∼167 nm. The calculations reproduce experimental absorption cross-sections at a resolution of ∼0.5 nm without scaling the intensity. The observed structure in the 150- to 210-nm range is caused by excitation of bending motion supported by the deep wells at bent geometries in the and potential energy surfaces. Predissociation below the singlet channel threshold occurs via spin-orbit coupling to nearby repulsive triplet states. Carbon monoxide vibrational and rotational state distributions in the singlet channel as well as the triplet channel for excitation at 157 nm satisfactorily reproduce experimental data. The cross-sections of individual CO2 isotopologues (12C16O2, 12C17O16O, 12C18O16O, 13C16O2, and 13C18O16O) are calculated, demonstrating that strong isotopic fractionation will occur as a function of wavelength. The calculations provide accurate, detailed insight into CO2 photoabsorption and dissociation dynamics, and greatly extend knowledge of the temperature dependence of the cross-section to cover the range from 0 to 400 K that is useful for calculations of propagation of stellar light in planetary atmospheres. The model is also relevant for the interpretation of laboratory experiments on mass-independent isotopic fractionation. Finally, the model shows that the mass-independent fractionation observed in a series of Hg lamp experiments is not a result of hyperfine interactions making predissociation of 17O containing CO2 more efficient.