On the Formation of (Anionic) Excited Helium Dimers in Helium Droplets

Abstract

Metastable atomic and molecular helium anions exhibiting high-spin quartet configurations can be produced in helium droplets via electron impact. Their lifetimes allow detection in mass spectrometric experiments. Formation of atomic helium anions comprises collision-induced excitation of ground state helium and concomitant electron capture. Yet the formation of molecular helium anions in helium droplets has been an unresolved issue. In this work, we explore the interaction of excited helium atoms exhibiting high-spin triplet configurations with ground state helium using the equation-of-motion coupled-cluster method. Transition barriers in the energetically lowest He*–He and He*––He interaction potentials prevent molecule formation at the extremely low temperatures present in helium droplets. In contrast, some excited states allow a barrier-free formation of molecular helium (anions). Moreover, we show that the necessary excitation energies pinpoint (higher) resonances in recently recorded mass spectra and emend the assignment of those resonances that have previously been assigned to electron-impact ionization of ground state helium necessitating subsequent double-electron capture. Embedding molecules or molecular clusters in helium droplets is a predestined experimental technique for the study of phenomena at very low temperatures. Profound knowledge about active processes in the helium environment is required for a proper assessment of experimental data.

Ion formation upon electron collisions with valine embedded in helium nanodroplets

“…Anion formation at low electron energies is strongly suppressed for large HNDs in comparison to smaller droplets [39,47,50,51]. We explain this (i) by the low mobility of the electron bubble [43,[55][56][57] and (ii) by the reduced ejection probability of parent cluster anions of amino acids in case they are formed. Also, in the energy range of DEA that is typically assigned to core excited resonances, the fragmentation pattern differs strongly between doped HNDs and the gas phase.…”

“…In contrast, the heliophilic He* − is highly mobile and efficiently interacts also with heliophilic dopants. The electronic energy of He* − (19.74 eV) is only slightly lower than that of He* [43] and thus all dopants, except Ne, can become positively charged via reaction (1).…”

“…− [42,43]. In Figure 7 the total anion yield (solid black line) is plotted in comparison to the anion yield of He* − from pristine (red dashed line, data taken from [42] and SF 6 doped (blue dash dotted line, data taken from [52]) HNDs and the anion efficiency curve of (C 70 )…”

A configurational study of helium clusters doped with He∗− and He2∗−

“…1 The excitation to metastable electronic states of the helium clusters was taken as the main mechanism producing He anions instead of direct interaction of electrons with the neutral He droplet. [1][2][3] In fact it is now well accepted that the process which leads to the formation of atomic helium anions is the result of a two-step reaction. First, the electron penetrating inside the helium droplet excites a He atom from the ground electronic state He(1s 2 1 S) to its first excited state He(1s2s 3 P), namely, He * .…”

“…2 The above mentioned electron energy required to penetrate the helium droplet summed to the corresponding excitation energies lead to estimates for the formation energy of He * − 2 of 22.1 eV and 24.2 eV, respectively. 2,3 Both species He * − and He * − 2 show different behaviour once they become embedded in the droplet. On one hand, the atomic anion is highly mobile through the surrounding helium environment and has been suggested as the effective charge carrier in the formation of anions inside helium droplets instead of the previously assumed possibility of an electron bubble migrating to the dopant.…”