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研究分子结中电流对入射激光脉冲的时间依赖响应是获取分子结构及其激发态信息的重要途径.本文构建了一个由金属电极/聚乙炔分子/金属电极构成的有机分子结,采用Su-Schrieffer-Heeger模型描述分子结构,结合非平衡格林函数方法和级联运动方程,求解了高斯型激光脉冲作用下分子结的瞬态响应特性.研究发现,激光脉冲的中心频率是影响瞬态电流响应特性的关键因素之一.仅当电子与光场发生共振时,才能引起大量电荷的激发.由于电子-声子耦合作用,被激发的电子引发晶格弛豫并形成激子态,导致能量转移,使得电流振幅显著增大,频谱中特征峰分布区域变宽.冻结晶格原子运动后,发现电流振幅减小,电流频谱变得简洁,验证了电子-声子耦合是分子结内光诱导电流的另一个关键因素.这些发现有助于理解分子对外部光刺激的动态响应机制,也为设计新型光电分子器件提供理论基础.The time-dependent response of transient current to incident laser pulses in molecular junctions is an important method to obtain information about molecular structures and excited-state dynamics. In this work, a theoretical study is carried out on the transient charge transport through a model polyacetylene molecular junction driven by Gaussian-type femtosecond laser pulses. The molecule is described by the extended Su-Schrieffer-Heeger model, which explicitly includes electron-phonon interactions and captures both electronic and lattice degrees of freedom. The transient transport dynamics are calculated by combining the non-equilibrium Green’s function formalism with the hierarchical equations of motion, allowing a fully non-adiabatic description of the coupled electron-lattice evolution.
Results show that the central frequency of the incident laser pulse is one of the key factors that determines the transient current response. When electrons resonate with the optical field, the current amplitude is significantly enhanced, and the temporal profile becomes asynchronous with the laser field, indicating strong non-linear response. The corresponding current spectra exhibit broadened main peaks accompanied by multiple sidebands, suggesting the coexistence of various frequency components due to dynamic coupling between electrons and lattice vibrations.
Further analysis of the evolution of instantaneous energy levels demonstrates that, under resonant excitation, electrons are efficiently excited from HOMO to LUMO. The excited electrons induce lattice relaxation through electron-phonon coupling, resulting in local structural distortion and the formation of self-trapped excitonic states. These excitonic effects lead to additional energy transfer channels, thus amplifying the current response and broadening the frequency spectrum.
In contrast, when the lattice motion is artificially frozen, both the current amplitude and frequency broadening are greatly suppressed, and only a single sharp spectral peak corresponding to the laser frequency is observed. This comparison clearly demonstrates that electron-phonon coupling is a key factor governing the transient transport behavior in molecular junctions under optical excitation.
The present study reveals the microscopic mechanism of light-induced transient transport in organic molecular junctions and highlights the essential role of lattice dynamics in modulating non-equilibrium charge transfer. These findings provide theoretical guidance for the design of novel optoelectronic molecular devices and contribute to the fundamental understanding of non-adiabatic transport processes in low-dimensional quantum systems.-
Keywords:
- Molecule junction /
- Electron-phonon coupling /
- Transient transport /
- Light-induced current
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