Theoretical Studies of Electron Dynamics in Nanometer-Sized Molecules

分子系统中的动力学可分为两类：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，电子动力学与分子结中的线性和非线性光学响应、电子电荷转移或电子传导有关。电子动力学是一个超快的过程，很难精确地考虑电子的关联，因此，尽管电子动力学具有重要意义，但电子动力学还没有得到理论上的研究。我们从理论上证明了圆偏振激光脉冲在环形分子Na_2和苯中感应出电流和磁矩<10>。采用含时绝热局域密度近似，在实空间和实时求解含时Kohn-Sham方程。已经发现，电流被有效地诱导，并且即使在激光脉冲被切断之后也持续存在，只要所施加的激光脉冲的频率与具有每个分子系统的偶极强度的电子激发态的激发能量一致。实验结果表明，电流是对电场强度的二阶非线性光学响应。磁偶极矩不可避免地伴随着环电流，因此分子被磁化。电流和磁矩在本程序中的生产被发现是更有效的比利用静态磁场。然后，我们发展了一种基于量子刘维尔方程的数值方法，以讨论量子耗散系统中的电子动力学。

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.