Electron Diffraction of Ionic Argon Nanoclusters Embedded in Superfluid Helium Droplets

Abstract

We report electron diffraction of cationic argon nanoclusters embedded in superfluid helium droplets. Superfluid helium droplets are first doped with neutral argon atoms to form nanoclusters, and then the doped droplets are ionized by electrons. The much lower ionization energy of argon ensures that the positive charge resides on the Ar nanocluster. Using different stagnation temperatures therefore droplets with different sizes, we have been able to preferentially form a small ionic cluster containing 2 -4 Ar atoms and a larger cluster containing 7 -11 atoms. The fitting results of the diffraction profiles agree with structures reported from theoretical calculations, containing a cationic trimer core with the remaining atoms largely neutral. This work testifies to the feasibility of performing electron diffraction from ionic species embedded in superfluid helium droplets, dispelling the concern over the particle density in the diffraction region. However, the large number of neutral helium atoms surrounding the cationic nanoclusters poses a challenge for the detection of the helium solvation layer, and the detection of which awaits further technological improvements.

Electron diffraction as a structure tool for charged and neutral nanoclusters formed in superfluid helium droplets

“…The experimental apparatus is shown in Fig. 1, 47–49 including a superfluid helium droplet source, a pickup and cluster formation chamber, and a diffraction chamber. For studies of ionic species, an ionization and ion control component is also needed.…”

“…For experiments of ionic samples, neutral cluster doped droplets can be ionized by electron impact ionization (EII) or laser ionization. 47,64 Hot electrons are accelerated to a biased central grid surrounding the droplet beam, and ionized droplets are accelerated as they travel out of the grid into the surrounding at ground potential. 61 The much lower ionization energy of any dopant molecule ensures that the final location of the charge rests on the dopant cluster.…”

“…In experiments of ionized clusters, 47,64 both the DPV and SPV operate at 10 Hz, and the distinction between the “signal” and “background” relies on the status of the pulsed kicker electrode, which operates at 5 Hz. The “signal” is therefore from accelerated ions, and the “background” is from the neutral ambient without acceleration.…”

“…We have recorded diffraction profiles of molecular clusters formed inside helium droplets, both from neutral and ionized species. 47,51,55,64,69 Limited by the doping method, the nanoclusters are polydisperse, which offers a window to observe the structure evolution with cluster sizes. For neutral clusters, so far all of our samples approach bulk crystal structures when more than four monomers are present.…”

“…For neutral clusters, so far all of our samples approach bulk crystal structures when more than four monomers are present. 47,51,69 When the global minimum structure in the gas phase deviates from a cut of the crystal structure, some small clusters adopt the structures in the gas phase. 47,51,55,64 However, the situation is not universal, and for example, in the case of iodine dimers, 69 a halogen bond is already formed, analogous to the most compact dimeric cut from bulk solids.…”