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.