低能Cl-离子在绝缘纳米微孔膜中的传输过程

理论模拟结合实验研究了16-keV Cl-离子穿越不同厚度(7和12 μm)的Al₂O₃微孔膜的物理过程，发现负离子传输中并不存在与正离子传输类似的明显的导向现象。在只考虑散射过程的情况下，模拟出的穿透粒子角分布及电荷态分布与实验结果符合很好，出射的Cl-离子沿初束方向分布；Cl0、Cl+离子沿微孔轴向分布。仔细分析了不同出射粒子的角分布，发现出射的Cl+在微孔轴向与初束方向之间分布；经单次散射出射的Cl0沿微孔轴向分布，而经多次散射出射的Cl0向初束方向移动。发现了Cl-离子穿越不同厚度的具有相同微孔直径的Al₂O₃微孔时，较厚的膜出射的Cl+/Cl0比例低。理论分析显示，这是由散射过程的特性造成的，随着微孔膜厚度的增加，出射的Cl0中经单次碰撞的比例变小，而多次散射出射的比例增加，从而导致Cl+离子转化为Cl0的几率要远大于Cl0转化为Cl+离子的几率，使得长的微孔出射的粒子中Cl+/Cl0比例低。

Transmission of Low-energy Cl- through Insulating Nanocapillaries

The transmission of 16-keV Cl– ions through Al₂O₃ nanocapillaries of 7 and 12 μm in thickness was studied both by experiment and simulation. It is found that the transmission of negative ions is different from that of positive ions through insulating nanocapillaries, where the deposited charges result in the so-called guiding effect. For the case of only the scattering, the transmitted angular distributions and charge state distributions from the simulations agreed well with the experimental results, i.e., the transmitted Cl– ions exits to the direction of the primary beam; the transmitted Cl0 and Cl– spread around the axis of the capillaries. The analysis of the simulated trajectories shows that the transmitted Cl+ ions spans from the axis of nanocapillaries to the primary beam direction; the transmitted Cl0 due to the single scattering is centered around the axes of the nanocapillaries while the transmitted Cl0 through multiple scattering shifts to the direction from the axes of the capillaries to the primary beam direction. From the simulations, it is found that the ratio of Cl+/Cl0 for the transmitted particles exited from the nanocapillaries of longer lengths is lower, in accord to the experiments. The increase of the length of the capillaries will lead to the drop of the portion of transmitted Cl0 by single scattering and the increase of the probability of the exiting of Cl0 through multiple scatterings. Therefore the probability of Cl+ ions changed to Cl0 is much larger than that Cl0 changed to Cl+ ions in the collision process, leading to the smaller ratio of Cl+/Cl0 for the transmitted particles exited from the nanocapillaries of longer lengths.