弱耦合近似下激子-激子湮灭动力学研究

Theoretical study of exciton-exciton annihilation dynamics in the approximation of weak coupling

分子聚集体中激子-激子湮灭动力学过程与其发光特性和能量转移过程密切相关,研究激子动力学过程对模拟自然界中的光合作用以及研究其光载流子的输运过程有重要意义.本文在密度矩阵理论框架下引入弱耦合近似得到率方程,用率方程研究了分子间距离、激子态密度和激发态偶极矩与激子-激子湮灭动力学的关系.研究发现当分子间距离减小时,第一激发态衰变过程受激子融合过程的影响,导致其衰变有明显的S型特征.高阶激发态的偶极矩是激子融合过程的关键因素,且偶极矩越大,激子融合过程越容易发生.在不同激子密度下第一激发态随时间的演变规律与在低功率下激发聚对苯乙炔(OPPV7)类单体和不同功率下激发聚对苯乙炔(OPPV7)类聚集体的激子动力学过程的实验结果一致.考虑光激发作用下的量子波包作为初始态,模拟了不同激子能级下的激子动力学演变过程发现激子态在几百飞秒内能够保持很好的局域性,说明激子态是相干叠加态,其局域特点与所在的激发能级有关.

Dynamics of exciton-exciton annihilation(EEA)in molecular aggregates is closely related to its luminescence characteristics and energy transfer.It is meaningful to uncover energy and charge transfer process in molecular systems.Therefore,studying the dynamics of exciton is important for simulating photosynthesis in nature and analyzing the transport process of photocarriers.In this paper the weak coupling approximation is adopted to obtain the rate equation in the framework of density matrix theory.The relation among the intermolecular distance,exciton state density,excited state dipole moment and exciton-exciton annihilation dynamics is studied by the rate equations.It is found that the decrease of intermolecular distance leads the generation rate of higher-order excited states to increase,resulting in the obvious S-shaped decay characteristics.Moreover,the dipole moment of the higher-order excited state is the key factor of the exciton fusion process,and the greater the exciton density,the more easily the exciton fusion process occurs.Therefore,the reduction of intermolecular distance and the increase of the dipole moment of the higher-order excited state make the nearest neighbor molecules have a strong coupling,resulting in a high generation rate of the higher-order excited state.It is found that the evolution processes of the first excited state in different exciton densities are consistent with the experimental results of the excitation of OPPV7 monomer(PPV oligomers of 7)at a low excitation energy,and the excitation of OPPV7 aggregates at different excitation energy levels.It can be observed that the exciton decay rate is faster under the excitation of the strong external field.Using the quantum wave packet under optical excitation as the initial state,the excited state dynamics is simulated at different exciton energy levels.It is found that the exciton state can maintain good locality within a few hundreds of femtoseconds,which shows that the exciton state is a coherent superposition state,and its local characteristics are related to the excitation energy level.These conclusions are applicable to the aggregations whose single molecule has an energy level of E_(mf)≈2E_(me) ,and also provide a reasonable reference for the exciton-exciton annihilation process under optical excitation.