Eur. Phys. J. D (2016) 70: 148
https://doi.org/10.1140/epjd/e2016-70136-3

Regular Article

Helium anion formation inside helium droplets^*

Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria

^a e-mail: stephan.denifl@uibk.ac.at

Received: 26 February 2016
Received in final form: 20 May 2016
Published online: 5 July 2016

Abstract

The formation of He^∗− is examined with improved electron energy resolution of about 100 meV utilizing a hemispherical electron monochromator. The work presented provides a precise determination of the three previously determined resonance peak positions that significantly contribute to the formation of He^∗− inside helium nanodroplets in the energy range from 20 eV to 29.5 eV. In addition, a new feature is identified located at 27.69 ± 0.18 eV that we assign to the presence of O₂ as a dopant inside the droplet. With increasing droplet size a small blue shift of the resonance positions is observed. Also for the relatively low electron currents used in the present study (i.e., 15–70 nA) a quadratic dependence of the He^∗− ion yield on the electron current is observed.