Classification of doubly excited molecular electronic states.
Classification of doubly excited molecular electronic states.
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DOI：
10.1039/d2sc06990c
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发表时间：
2023-04-12
期刊：
影响因子：
8.4
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中科院分区：
文献类型：
--
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具有部分或完全双激发性质的电子态在单线态裂变和非线性光学光谱等许多领域中起着至关重要的作用。尽管多烯及相关体系中的双激发态已被研究多年，但即使在最简单的丁二烯情况下，单激发态与双激发态的归属也引发了争议。到目前为止，对于双激发态的分类还没有一个明确的框架，更重要的是，甚至还没有一个被广泛接受的双激发态特征的定义。在这里，我们提出了一个解决方案：基于算子期望值和密度矩阵的双激发特征的物理激励定义，其独立于底层轨道表示而工作，避免了困扰早期研究的歧义。此外，我们提出了一种分类方案来区分三种情况：(I)两个单独的激发发生在两个独立的轨道对内，留下四个开放壳层(DOS)，(Ii)两个电子提升到同一轨道，产生闭壳行列式(DC)，以及(Iii)单激发和双激发组态的混合，与前面任何一种情况(DMix)都不一致。我们强调了它们在潜在能量项上的差异，并在实际计算中解释了它们的特征。通过不同的高级计算方法说明了这三种情况，分别使用二聚体(DOS)、多烯(DMix)和环丁烷和四嗪(DCSs)。利用一种著名的光化学反应--乙烯的光二聚反应，研究了DOS和DCS之间的转化。这项工作提供了对双激发态更深入的理解，并可能指导更严格的讨论，以改进它们的计算描述，同时也给出对它们的基本光物理的洞察。提出了一种严格的双激发态分类方案，并以不同的分子为例说明了不同的可能类型。
Electronic states with partial or complete doubly excited character play a crucial role in many areas, such as singlet fission and non-linear optical spectroscopy. Although doubly excited states have been studied in polyenes and related systems for many years, the assignment as singly vs. doubly excited, even in the simplest case of butadiene, has sparked controversies. So far, no well-defined framework for classifying doubly excited states has been developed, and even more, there is not even a well-accepted definition of doubly excited character as such. Here, we present a solution: a physically motivated definition of doubly excited character based on operator expectation values and density matrices, which works independently of the underlying orbital representation, avoiding ambiguities that have plagued earlier studies. Furthermore, we propose a classification scheme to differentiate three cases: (i) two single excitations occurring within two independent pairs of orbitals leaving four open shells (DOS), (ii) the promotion of both electrons to the same orbital, producing a closed-shell determinant (DCS), and (iii) a mixture of singly and doubly excited configurations not aligning with either one of the previous cases (Dmix). We highlight their differences in underlying energy terms and explain their signatures in practical computations. The three cases are illustrated through various high-level computational methods using dimers for DOS, polyenes for Dmix, and cyclobutane and tetrazine for DCS. The conversion between DOS and DCS is investigated using a well-known photochemical reaction, the photodimerization of ethylene. This work provides a deeper understanding of doubly excited states and may guide more rigorous discussions toward improving their computational description while also giving insight into their fundamental photophysics. A rigorous classification scheme for doubly excited states is proposed and the different possible types are exemplified in various molecules.
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