Dynamic evolution of 100-keV H+ through polycarbonate nanocapillaries

Abstract

In recent years, the guiding effect of highly charged ions (HCIs) through insulating nanocapillary membrane has received extensive attention. It is found that slow highly charged ions at keV energies can be guided along the capillary even when the title angle of membrane is a few degrees and larger than geometry opening angle of the capillary. Initially, Stolterfoht et al. (2002 Phys. Rev. Lett. 88 133201), according to the incident ions deposit positive charges on the capillary surface in a self-organizing manner, proposed scattering and guiding regions to explain this guiding phenomenon. Hereafter, a detailed experiment and simulation performed by Skog et al. (2008 Phys. Rev. Lett. 101 223202) provided clear evidence that the guiding process is actually attributed to the self-organized charge patches formed on the inner capillary walls. HCIs entering into a capillary may hit the surface, leaving their charge on the inner wall of the capillary. When the capillary axis is tilted with respect to the beam incidence direction, a charge patch is formed in the capillary entrance, simultaneously a repulsive electric field is created. After sufficient charge deposition this field is strong enough to deflect the subsequent ions in the direction of the capillary exit. Therefore, the ions are guided through the capillary. The deflection at the charge patch only occurs at relatively large distances from the capillary wall so that the incident charge state of the ions is kept during the passage through the capillary. Further experimental and theoretical studies on various target materials, such as polyethylene terephthalate (PET), polycarbonate (PC), SiO2, and Al2O3 indeed found that a single or a small number of charge patches near the entrance formed in the charge up process dominate the observed oscillatory variations of the ion emission angle and the final guiding process. Besides, measurements and simulations of the steering of swift ions at MeV energies have shown that the transmission mechanism of the high energy ions in a tapered tube is primarily dominated by multiple random inelastic collisions below the surface and the charge patches are not responsible for the transmission process. However, the studies of the transmission of hundreds keV ions through nanocapillaries are still lacking so far. In this work, we observe the evolution of the angular distribution, charge state distribution, FWHM and transmission rate of 100 keV H+ ions incident on a polycarbonate (PC) membrane at +1 tilt angle. It is found that the transmitted particles are located around the direction along the incident beam, not along the capillary axis, which suggests that the mechanism of hundreds keV (E/q~100 kV) protons through capillaries is significantly different from that for the guiding effect of keV protons. We present a qualitative explanation based on the data: that the 100 keV H+ are transmitted by multiple random inelastic collisions below the surface is attributed to the absence of the deposited charges on the surface of the capillary at the beginning of the experiment. After the equilibrium, several charge patches are formed on the inner wall of the capillary, which suppresses the ions to penetrate into the surface of the capillary, while the H+ is transmitted via specular scattering above the surface (or closest to the surface) assisted by the charge patches, and finally is emitted in the incident direction through twice specular scattering. This finding increases the knowledge of charged ions through nanocapillaries, which is conducible to the applications of nanosized beams produced by capillaries or tapered glass within hundreds keV energies in many scientific fields.