Transmission of low-energy negative ions through insulating nanocapillaries

Abstract

A simulation is performed to study the transmission of low-energy $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$ ions through $\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ nanocapillaries. For the trajectory simulations, there are several processes involved: the image forces induced by the projectile; the electrostatic force from the deposited charges; the scattering from the inner surface and charge exchange. The simulation reproduces the main features of the experiments; i.e., the double peak structure in the transmitted angular distribution and the transmitted fractions of $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$, $\mathrm{C}{\mathrm{l}}^{+}$, and $\mathrm{C}{\mathrm{l}}^{0}$ were found in the charge state distribution. The transmitted $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$ ions are centered around the beam direction while the transmitted fractions of $\mathrm{C}{\mathrm{l}}^{0}$ and $\mathrm{C}{\mathrm{l}}^{+}$ are centered around the tilt angles. The role of the deposited charge is also studied by simulations. With the deposited charge, it is found that $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$ is dominant in the transmission and the majority of the ions, centered around the tilt angle, are mainly from the single deflection by the negative charge patches on the inner surfaces of the capillaries, and only a few directly transmitted $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$ ions are centered around the incident direction. There are also a few transmitted fractions of $\mathrm{C}{\mathrm{l}}^{0}$ and $\mathrm{C}{\mathrm{l}}^{+}$ from close surface scatterings. In the case that there are no negative charge patches, the simulation agrees with the experiment in detail: The majority of the directly transmitted $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$ ions are centered around the incident direction while only a few scattered $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$ ions are centered around the tilt angle from the single close collisions with the inner surfaces of the capillaries. There is a portion, comparable to the transmitted fraction of $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$, of the transmitted fractions of $\mathrm{C}{\mathrm{l}}^{0}$ and $\mathrm{C}{\mathrm{l}}^{+}$, centered around the tilt angle, from the single scatterings with the inner surfaces of the capillaries. This confirms that at the present experimental conditions there are most probably no negative charge patches formed to guide the negative ions through insulating $\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ nanocapillaries.