Photoabsorption and photodissociation in molecular nitrogen
Photoabsorption and photodissociation in molecular nitrogen
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分子氮的光吸收和光解离
DOI：
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发表时间：
2010
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影响因子：
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通讯作者：
A. Heays
中科院分区：
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作者：
A. Heays
通过耦合薛定谔方程的求解，精确模拟了分子氮在极紫外波长下的光吸收和光解离，以阐明 N2 的光谱和预解离动力学及其光化学的实际应用。物理真实模型能够可靠地外推到其构建所需的光谱信息数据库之外，外推到 100 000 至 118 000 cm−1 范围内的能量，并且有一些注意事项，超出此范围；以及 N2 的任何温度、旋转状态和同位素异构体。该模型模拟光谱具有有效无限的分辨率，并再现所有电偶极子允许的 Πu 和 Σu 态的旋转能级能量及其吸收 f 值，在整个范围内达到光谱精度。计算出的转变的预解离线形可以准确地再现，就像背景连续体一样，即使在跨越解离极限的情况下也是如此。高度扰动的频谱被证明是由多通道效应引起的，多通道效应只能通过耦合处理来再现，其中包括同质和异质相互作用的效应。模型公式中允许存在未结合的解离态，并且 Πu 和 Σu 态的复合体被证明是通过自旋轨道相互作用导致单重态能级的预解离的原因。对于所有同位素异构体和高达 111 000 cm−1 的能量，三重态的光谱参数以及 Πu 预解离率在多个数量级上的变化已被精确建模。将计算出的解离波函数转换为渐近明确定义的原子态的基础，可以计算它们的解离前分支比。对耦合态的势能曲线、混合它们的非对角矩阵元素以及负责其光学激发的电子跃迁矩（包括在某些情况下它们对核间距离的依赖性）进行了新的、准确的测定。已经对许多转变进行了旋转分辨绝对 f 值和预解离线宽的新支持实验室测量，其中一些转变以前从未观察到。这些不同的实验采用同步加速器和激光辐射以及电子碰撞激发，并通过光栅光谱仪、傅里叶变换光谱仪和光碎片检测进行检测。耦合通道建模促进了这些研究的分析，然后通过产生的新数据来通知该模型。耦合通道模型的特征非常适合行星大气光化学建模者对 N2 光谱的迫切需求，因为它可以现实地扩展到可变的现实世界条件。这里介绍了该模型在土卫六大气中的应用，它解释了在 N2 光吸收细节方面观察到的不寻常的氮同位素比率。这里介绍了建模光谱的另一种用途，用于分析陆地日光。耦合通道模型的适用性相当广泛，利用它的进一步研究正在进行中。
The photoabsorption and photodissociation of molecular nitrogen at extreme-ultraviolet wavelengths has been precisely modelled by solution of the coupled Schrodinger equation, for the purposes of elucidating the spectroscopy and predissociation dynamics of N2 and for practical application to its photochemistry. The physically realistic model is capable of reliable extrapolation beyond the database of spectroscopic information necessary for its construction, to energies in the range of 100 000 to 118 000 cm−1 and, with some caveats, beyond this; and for any temperature, rotational state, and isotopomer of N2. The model simulated spectra have an effectively-infinite resolution, and reproduce the rotational level energies of all electric-dipole-allowed Πu and Σu states, and their absorption f -values, to spectroscopic accuracy over its entire range. The predissociated lineshapes of calculated transitions are accurately reproduced, as is the background continuum, even where a dissociation limit is crossed. The highly perturbed spectrum is shown to arise from multi-channel effects which can only be reproduced by a coupled treatment which includes the effects of homogeneous and heterogeneous interactions. Unbound dissociative states are permitted in the model formulation and a complex of Πu and Σu states is shown to be responsible for the predissociation of singlet levels via spin-orbit interaction. The spectroscopic parameters of triplet states, and the variation of Πu predissociation rates over multiple orders of magnitude has been accurately modelled for all isotopomers and energies up to 111 000 cm−1. The transformation of the calculated dissociative wavefunctions into a basis of asymptotically well-defined atomic states allows for the calculation of their predissociation branching ratios. New and accurate determinations have been made of potential-energy curves for the coupled states, the off-diagonal matrix elements that mix them, and the electronic transition moments responsible for their optical excitation, including, in some cases, their dependence on internuclear distance. New supporting laboratory measurements of rotationally-resolved absolute f -values and predissociation linewidths have been made for many transitions, some of which have not been previously observed. These various experiments employ synchrotron and laser radiation as well as electron-impact excitation, and make detections by means of a grating spectrometer, Fourier-transform spectrometer, and the detection of photofragments. Analysis of these studies is facilitated by the coupled-channels modelling, which is then informed by the resulting new data. The characteristics of the coupled-channels model are an ideal match to the immediate need for N2 spectra by photochemical modellers of planetary atmospheres, because of its realistic extensibility to variable real-world conditions. An application of the model to the atmosphere of Titan is presented here, which explains the unusual observed ratio of nitrogen isotopes in terms of the details of N2 photoabsorption. Another use of modelled spectra is presented here, for the analysis of the terrestrial dayglow. The applicability of the coupled-channels model is quite broad and further investigations which utilise it are underway.