On Subthreshold Ionization of Helium Droplets, Ejection of He+, and the Role of Anions

TL;DR

This paper investigates how helium ions are formed and ejected from helium droplets at electron energies below the atom's ionization threshold, highlighting the role of metastable anions in this process.

Contribution

It introduces a model explaining subthreshold helium ionization involving metastable $He^{*-}$ anions, which was previously not well understood.

Findings

Metastable $He^{*-}$ anions are crucial for subthreshold ionization.

Ion yields depend on electron energy, current, and droplet size.

The proposed model is testable and advances understanding of helium droplet ionization.

Abstract

The mechanism of ionization of helium droplets has been investigated in numerous reports but one observation has not found a satisfactory explanation: How are $He^+$ ions formed and ejected from undoped droplets at electron energies below the ionization threshold of the free atom? Does this path exist at all? A measurement of the ion yields of $He^+$ and $He_2^+$ as a function of electron energy, electron emission current, and droplet size reveals that metastable $He^{*-}$ anions play a crucial role in the formation of free $He^+$ at subthreshold energies. The proposed model is testable.