氰基多炔负离子的束缚态和连续嵌入态
Bound and continuum-embedded states of cyanopolyyne anions
摘要：
氰基多炔阴离子是在星际介质中发现的最早的阴离子之一。这一发现提出了关于这些阴离子在太空中形成途径的问题。一些提出的机制假设，阴离子激发电子态，无论是亚稳态或弱结合，在形成过程中发挥了关键作用。验证这一假设需要详细了解阴离子的电子状态。本文用从头计算方法研究了四种氰基聚炔阴离子CN $^-$、C $_3$ N $^-$、C $_5$ N $^-$和C $_7$ N $^-$的束缚态和连续态.我们采用运动方程耦合团簇方法与复吸收势增强。我们预测，CN $^-$，在家庭中最小的阴离子，已经有几个低洼的亚稳态的单重态和三重态自旋对称性。这些状态被识别为形状共振，位于阴离子的基态以上6.3-8.5 eV之间（或母自由基的基态以上2.3-4.5 eV），并且具有十分之几eV至1 eV的宽度。我们根据主要分子轨道贡献和戴森轨道来分析所识别的共振。随着C $_{2n+1}$ N $^-$系列中碳链长度的增加，这些共振逐渐变得稳定，并最终变成稳定的价键态。导致共振态和束缚激发态的跃迁能量的趋势可以通过休克尔模型合理化。除了价激发态，一些氰基多炔也可以支持偶极束缚态和偶极稳定的共振，由于在最低的$^2Σ^+$状态的母自由基的大的偶极矩。我们讨论了中性自由基的开壳层性质对相应阴离子的偶极稳定态的影响。
英文摘要：
Cyanopolyyne anions were among the first anions discovered in the interstellar medium. The discovery have raised questions about routes of formation of these anions in space. Some of the proposed mechanisms assumed that anionic excited electronic states, either metastable or weakly bound, play a key role in the formation process. Verification of this hypothesis requires detailed knowledge of the electronic states of the anions. Here we investigate bound and continuum states of four cyanopolyyne anions, CN$^-$, C$_3$N$^-$, C$_5$N$^-$, and C$_7$N$^-$, by means of ab initio calculations. We employ the equation-of-motion coupled-cluster method augmented with complex absorbing potential. We predict that already in CN$^-$, the smallest anion in the family, there are several low-lying metastable states of both singlet and triplet spin symmetry. These states, identified as shape resonances, are located between 6.3-8.5 eV above the ground state of the anion (or 2.3-4.5 eV above the ground state of the parent radical) and have widths of a few tenths of eV up to 1 eV. We analyze the identified resonances in terms of leading molecular orbital contributions and Dyson orbitals. As the carbon chain length increases in the C$_{2n+1}$N$^-$ series, these resonances gradually become stabilized and eventually turn into stable valence bound states. The trends in energies of the transitions leading to both resonance and bound excited states can be rationalized by means of the Hückel model. Apart from valence excited states, some of the cyanopolyynes can also support dipole bound states and dipole stabilized resonances, owing to a large dipole moment of the parent radicals in the lowest $^2Σ^+$ state. We discuss the consequences of open-shell character of the neutral radicals on the dipole-stabilized states of the respective anions.
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