What is wrong with the image charge force of keV ions in insulating nano-capillaries

TL;DR

This paper investigates the discrepancy between theoretical and experimental transmitted fractions of keV ions in nano-capillaries, revealing that velocity-dependent image charge forces may explain the differences.

Contribution

It introduces exact expressions for velocity-dependent image charge forces at dielectric interfaces and re-evaluates ion transmission, addressing previous theoretical-experimental discrepancies.

Findings

Theoretical transmitted fractions are much lower than experimental data.

Velocity dependence of the image charge force can significantly affect ion transmission.

Re-evaluation with velocity-dependent forces offers better agreement with experiments.

Abstract

In nano-capillaries of large aspect ratio, the attractive image charge force is strong enough to affect the trajectory of ions passing through capillaries and consequently to diminish the fraction of transmitted beam ions. We calculated the theoretically transmitted fraction, using an approached but CPU-friendly expression of the image charge force valid in the case of a static ion and an infinite cylindrical dielectric interface. When comparing the theoretically transmitted fraction to available experimental data for nano-capillaries with an inner diameter of less than 200 nm, we found a surprisingly large disagreement, i.e., the theoretically transmitted fractions were easily an order of magnitude lower than the experimental ones. Noting that the image charge force depends on the velocity of the ion via the frequency dependent relative permittivity of the insulator, we investigated whether the disagreement could be lifted using a velocity depend image charge force. We give the exact expressions of the dynamical image charge force for a plane and cylindrical dielectric interface as a function of the ion velocity. We then re-evaluated the theoretically transmitted fractions in the case of SiO$_2$ and PET dielectric interfaces. Our findings are discussed in the light of the available experimental data.