Excitation mechanism of the mesospheric sodium nightglow
Excitation mechanism of the mesospheric sodium nightglow
复制标题
中层钠夜光的激发机制
作者：
D. Herschbach;C. Kolb;D. Worsnop;X. Shi
大气中的钠夜光和流星尾迹的发光，都发生在85-95千米的中间层高度，是由处于激发态2P的钠原子发出的。查普曼机制将这些归因于NaO和氧原子之间的反应，相对于NaO + O反应的基态Na（2S）原子，需要异常高的激发Na*(2P)形成速率。但是动力学的实验室研究表明Na*(2P)的形成速率非常低（分支比f< 0.01）。NaO本身是由钠原子与臭氧反应形成的。分子束实验和光电子能谱最近表明，该反应产生的主要是激发态（2Σ+） NaO，而不是在NaO + O动力学实验中研究的基态（2Π） NaO，从而提出了与查普曼机制明显差异的解决方案。通过扩展Bates和Ohja6所考虑的反应物和生成物电子态之间的对称关系，我们在这里证明了激发态NaO与氧原子的反应确实产生了与Chapman机制一致的分支比。我们推断，导致激发态Na*(2P)原子的NaO + O势能面涉及双重态而不是四重奏自旋构型，基态NaO的分支比f接近于零，激发态NaO的分支比f接近于2/3。如果实验证实，这一发现可能使钠夜光用作中间层臭氧浓度的定量测量5,7,8。
THE atmospheric sodium nightglow and luminescence of meteor trails, both of which occur at mesospheric altitudes of 85–95 km, are emitted by sodium atoms in the excited 2P state. The Chapman mechanism1–8 attributes these to the reaction between NaO and oxygen atoms, requiring an unusually high rate of formation of excited Na*(2P) relative to ground-state Na(2S) atoms from the NaO + O reaction. But laboratory studies of the kinetics9 show a very low Na*(2P) formation rate (branching ratio f< 0.01). NaO is itself formed by the reaction of sodium atoms with ozone. Molecular-beam experiments10 and photoelectron spectroscopy11 have shown recently that this reaction yields largely excited-state (2Σ+) NaO rather than the ground-state (2Π) species studied in the NaO + O kinetics experiments9, thereby suggesting a resolution of the apparent discrepancy with the Chapman mechanism. By extending the symmetry correlation between reactant and product electronic states considered by Bates and Ohja6, we show here that reaction of excited-state NaO with oxygen atoms does indeed yield branching ratios consistent with the Chapman mechanism. We infer that the NaO + O potential-energy surfaces leading to excited Na*(2P) atoms involve doublet rather than quartet spin configurations, and the branching ratio f is close to zero for ground-state NaO but ∼2/3 for excited-state NaO. If confirmed experimentally, this finding may enable the sodium nightglow to be used as a quantitative measure of mesospheric ozone concentrations5,7,8.