Helium anion formation inside helium droplets

Abstract

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 O 2 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.

Cold physics and chemistry: Collisions, ionization and reactions inside helium nanodroplets close to zero K

“…Inspection of the anion efficiency curve for forming He*  revealed a resonant feature around 22 eV which could be reproduced by a triple-Gaussian function, yielding maxima at 22.0  0.2 eV, 23.0  0.2 eV, and 25.1  0.5 eV. A later study utilizing a hemispherical electron monochromator with higher electron energy resolution confirmed the previous values (21.98  0.05 eV, 22.96  0.07 eV, and 25.78  0.1 eV [178]. Similar values have been reported earlier for the resonant formation of anionic HNDs [44] and also for anion formation of or from various dopants inside HNDs, e.g.…”

“…With the discovery of the efficient formation of He* -(for more details see Section 6) via a strong resonance at 22 eV and two weaker resonances at 23 eV and  25 eV [176][177][178], Renzler et al [175] proposed a mechanism for formation of He + at electron energies below the ionization energy of the helium atom that involves He* -. An incident electron with a kinetic energy of 22 eV can form He* and the inelastically scattered electron will then form a vacuum bubble via Pauli-repulsion, a so-called "electron-bubble" [179,180].…”

Electron Attachment and Electron Ionization of Formic Acid Clusters Embedded in Helium Nanodroplets

“…The resonance at 22.5 eV is a characteristic feature of doped HNDs, due to resonant formation of electronically excited metastable He* − anions. These ions are highly mobile in helium; they will be attracted towards the dopant by ion-induced dipole interaction followed by electron transfer to the dopant [ 26 , 27 ]. Therefore, it is not surprising that all anions shown in Figure 4 exhibit this resonance.…”