Takatsuka Kazuo, Arasaki Yasuki
Fukui Institute for Fundamental Chemistry, Kyoto University, 606-8103 Kyoto, Japan.
J Chem Phys. 2023 Aug 21;159(7). doi: 10.1063/5.0159178.
We study the chaotic, huge fluctuation of electronic state, resultant intramolecular energy redistribution, and strong chemical bonding surviving the fluctuation with exceedingly long lifetimes of highly excited boron clusters. Those excited states constitute densely quasi-degenerate state manifolds. The huge fluctuation is induced by persisting multidimensional nonadiabatic transitions among the states in the manifold. We clarify the mechanism of their coexistence and its physical significance. In doing so, we concentrate on two theoretical aspects. One is quantum chaos and energy randomization, which are to be directly extracted from the properties of the total electronic wavefunctions. The present dynamical chaos takes place through frequent transitions from adiabatic states to others, thereby making it very rare for the system to find dissociation channels. This phenomenon leads to the concept of what we call intramolecular nonadiabatic electronic-energy redistribution, which is an electronic-state generaliztion of the notion of intramolecular vibrational energy redistribution. The other aspect is about the peculiar chemical bonding. We investigate it with the energy natural orbitals (ENOs) to see what kind of theoretical structures lie behind the huge fluctuation. The ENO energy levels representing the highly excited states under study appear to have four robust layers. We show that the energy layers responsible for chaotic dynamics and those for chemical bonding are widely separated from each other, and only when an event of what we call "inter-layer crossing" happens to burst can the destruction of these robust energy layers occur, resulting in molecular dissociation. This crossing event happens only rarely because of the large energy gaps between the ENO layers. It is shown that the layers of high energy composed of complex-valued ENOs induce the turbulent flow of electrons and electronic-energy in the cluster. In addition, the random and fast time-oscillations of those high energy ENOs serve as a random force on the nuclear dynamics, which can work to prevent a concentration of high nuclear kinetic energy in the dissociation channels.
我们研究了电子态的混沌、巨大波动、由此产生的分子内能量重新分布，以及在高度激发的硼簇中具有极长寿命的强化学键在波动中得以存续的现象。这些激发态构成了密集的准简并态流形。这种巨大波动是由流形中各态之间持续的多维非绝热跃迁引起的。我们阐明了它们共存的机制及其物理意义。在此过程中，我们专注于两个理论方面。一是量子混沌和能量随机化，这将直接从总电子波函数的性质中提取。当前的动力学混沌通过从绝热态到其他态的频繁跃迁发生，从而使得系统极难找到解离通道。这种现象引出了我们所谓的分子内非绝热电子能量重新分布的概念，它是分子内振动能量重新分布概念在电子态上的推广。另一个方面是关于奇特的化学键。我们用能量自然轨道（ENO）来研究它，以了解巨大波动背后的理论结构。代表所研究的高激发态的ENO能级似乎有四个稳健的层次。我们表明，负责混沌动力学的能量层和负责化学键的能量层彼此广泛分离，只有当我们所谓的“层间交叉”事件突然发生时，这些稳健的能量层才会被破坏，导致分子解离。由于ENO层之间的能量间隙很大，这种交叉事件很少发生。结果表明，由复数值ENO组成的高能层会在簇中引发电子和电子能量的湍流。此外，那些高能ENO的随机且快速的时间振荡作为对核动力学的随机力，其作用是防止高核动能在解离通道中集中。
