On the properties of charged and neutral, atomic and molecular helium species in helium nanodroplets: interpreting recent experiments

Abstract

Properties of ground state He(=He(1s 2 1 S)), He + (=He(1s 2 S)), He + 2 (=He 2 (1σ 2 g 1σ u 2 + u )) and excited (metastable) He * (=He(1s2s 3 S)), He 2 * (=He 2 (1σ 2 g 1σ u 2σ g 3 + u )), He * − (=He(1s2s2p 4 P)) and He 2 * − (=He 2 (1σ 2 g 1σ u 2σ g 1π u 4 g )) are calculated using the coupled-cluster method and basis sets multiply augmented with diffuse functions. The aim of this work is to capture the essential physics needed to describe the qualitatively different behaviour of the above mentioned helium species dissolved in liquid helium. By studying their interaction with atomic ground state helium it is found that ground state He, He + , He 2 + and excited (metastable) He * − are well bound within a helium droplet. In comparison excited (metastable) He * , He 2 * and He 2 * − are found to be squeezed out due to the high energetic cost associated with the large volume they require inside a helium droplet. In particular, the molecular species He 2 * and He 2 * − consist of a positive core in the form of a He 2 + which is surrounded by a diffuse electronic cloud accounting for one or two electrons, respectively. The implications of these results for recent experimental studies on helium nanodroplets are discussed, particularly for the negatively charged species He * − and He 2 * − . We find that the latter species experience completely different dynamcis in a helium droplet although they are very similar in various other respects (e.g. diffuse electron clouds, size) in good agreement with experimental observations.

The interaction of He− with fullerenes

“…Having transferred the electrons at long range, the resulting He + can then combine with a nearby He atom to form He 2 + , which releases 2.5 eV of energy. [15][16][17] It is through this energy release that we account for the separation of the cation and the dianion. If all of the excess energy is deposited into kinetic energy, then we estimate that the two ions can escape Coulombic attraction even if electron transfer occurs at a distance as short as 12 Å.…”

“…17 In contrast, a fullerene cluster will be fully solvated by the helium and will locate itself somewhere near the center of the droplet. Consequently, in order for Penning ionization to take place, either the He * must migrate inwards or it must be formed by electron impact somewhere near the fullerene cluster, whereby it will then be drawn towards the fullerene by the polarizability of the latter.…”

“…We therefore conclude that the 2p 3 4 S state plays no role in our experiments, and therefore, the resonance in the He − state at 23 eV is attributed to an excited state of He − that has not been identified previously as a bound state. In a recent theoretical study, Huber and Mauracher suggested that the 1s2p 2 4 P state of He − might be responsible, 17 since it is expected to lie roughly 1 eV above the 1s2s2p 4 P state. This assignment seems plausible but it is surprising that the 1s2p 2 4 P state of He − has not been reported in earlier gas phase experiments.…”

“…One could attempt to identify other excited electronic states of He − to account for this feature but the challenge then would be to explain the long tail extending to much higher electron kinetic energies (>30 eV). Huber and Mauracher have suggested that this peak might have a very different source, arising from recombination of He + with two electrons, since the energy of the peak maximum lies only marginally above the ionization threshold of atomic helium in its ground state, 17 i.e., e − + He( 1 S) → 2e − + He + ( 2 S) → He − ( 4 P).…”

On subthreshold ionization of helium droplets, ejection of He+, and the role of anions

“…The highly polarisable He * and the electron bubble move towards each other and combine to form the heliophilic, highly mobile He *-in a quartet state. 43 The metastable ion is ejected if another incident electron is trapped in the droplet, thus explaining the approximately quadratic dependence of the He *-yield on the electron current. We adopt and extend the model summarized above to arrive at the following consistent explanation for the observation of bare He + at subthreshold energies: 1.…”