Theoretical Studies of Electron Dynamics in Nanometer-Sized Molecules

分子系统动力学分为两类：一类是核（或离子）动力学，另一类是电子动力学。化学反应动力学是核动力学的一个典型例子。另一方面，电子动力学与线性和非线性光学响应、电子电荷转移或分子结中的电子传导有关。尽管电子动力学具有重要的意义，但由于电子动力学是一个超快的过程，很难准确地考虑电子相关，因此尚无理论研究。我们从理论上证明了圆偏振激光脉冲在环形分子Na_<10>和苯中产生电流和磁矩。为此，采用时变绝热局部密度近似，在实空间和实时中求解时变Kohn-Sham方程。研究发现，只要所施加的激光脉冲频率与电子激发态的激发能和每个分子体系的偶极子强度相一致，即使在关闭激光脉冲后，也能有效地感应出电流并持续存在。电流对电场大小的二阶非线性光学响应被明确地揭示出来。磁偶极矩不可避免地伴随着环电流，因此分子被磁化。在本方法中，电流和磁矩的产生比利用静态磁场的产生要有效得多。为了讨论量子耗散系统中的电子动力学，我们进一步发展了一种基于量子Liouville方程的数值方法。

Dynamics in molecular systems is classified into two categories: one is nuclear (or ion) dynamics and the other one is electron dynamics. Chemical reaction dynamics is a typical example of the nuclear dynamics. On the other hand, the electron dynamics is related to linear and nonlinear optical response, electron charge transfer, or electronic conduction in molecular junctions. Despite their importance, the electron dynamics has not yet been studied theoretically because the electron dynamics is an ultrafast process and it is difficult to take account of electron correlation accurately. We have theoretically demonstrated that circularly polarized laser pulses induce electric currents and magnetic moments in ring-shaped molecules Na_<10> and benzene. The time-dependent adiabatic local density approximation was employed for this purpose, solving the time-dependent Kohn-Sham equation in real-space and real-time. It has been found that the electric currents are induced efficiently and persist continuously even after the laser pulses were switched off provided the frequency of the applied laser pulse is in tune with the excitation energy of the electronic excited state with the dipole strength for each molecular system. The electric currents were definitely revealed to be a second order nonlinear optical response to the magnitude of the electric field. The magnetic dipole moments inevitably accompany the ring currents, so that the molecules are magnetized. The production of the electric currents and the magnetic moments in the present procedure is found to be much more efficient than that utilizing static magnetic fields. We then proceeded to develop a numerical method based on quantum Liouville equation in order to discuss electron dynamics in quantum dissipative systems.