Theoretical Studies of Electron Dynamics in Nanometer-Sized Molecules

分子系统中的动力学分为两类：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。尽管它们很重要，但由于电子动力学是一个超快过程，很难准确地考虑电子关联，所以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状分子Na_lt；10&gt；和苯中感应电流和磁矩。为此，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。研究发现，只要激光脉冲的频率与每个分子体系的电子激发态的激发能量与偶极强度相一致，即使在激光脉冲被切断后，电流也能被有效地感应并持续存在。结果表明，电流对电场的大小具有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。发现在本过程中产生电流和磁矩比利用静磁场产生电流和磁矩效率高得多。然后，我们发展了一种基于量子Liouville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.