Electron Diffraction of Ionic Argon Nanoclusters Embedded in Superfluid Helium Droplets

Electron diffraction of foam-like clusters between xenon and helium in superfluid helium droplets

We report electron diffraction results of xenon clusters formed in superfluid helium droplets, with droplet sizes in the range of 105-106 atoms/droplet and xenon clusters from a few to a few hundred atoms. Under four different experimental conditions, the diffraction profiles can be fitted using four atom pairs of Xe. For the two experiments performed with higher helium contributions, the fittings with one pair of Xe-He and three pairs of Xe-Xe distances are statistically preferred compared with four pairs of Xe-Xe distances, while the other two experiments exhibit the opposite preference. In addition to the shortest pair distances corresponding to the van der Waals distances of Xe-He and Xe-Xe, the longer distances are in the range of the different arrangements of Xe-He-Xe and Xe-He-He-Xe. The number of independent atom pairs is too many for the small xenon clusters and too few for the large clusters. We consider these results evidence of xenon foam structures, with helium atoms stuck between Xe atoms. This possibility is confirmed by helium time-dependent density functional calculations. When the impact parameter of the second xenon atom is a few Angstroms or longer, the second xenon atom fails to penetrate the solvation shell of the first atom, resulting in a dimer with a few He atoms in between the two Xe atoms. In addition, our results for larger droplets point toward a multi-center growth process of dopant atoms or molecules, which is in agreement with previous proposals from theoretical calculations and experimental results.

Electron diffraction of 1,4-dichlorobenzene embedded in superfluid helium droplets

We perform electron diffraction of 1,4-dichlorobenzene (C6H4Cl2, referred to as 2ClB) embedded in superfluid helium droplets to investigate the structure evolution of cluster growth. Multivariable linear regression fittings are used to determine the concentration and the best model structures of the clusters. At a droplet source temperature of 22 K with droplets containing on average 5000 He atoms, the fitting results agree with the doping statistics modeled using the Poisson distribution: the largest molecular clusters are tetramers, while the abundances of monomers and dimers are the highest and are similar. Molecular dimers of 2ClB are determined to have a parallel structure with a 60° rotation for the Cl-Cl molecular axes. However, a better agreement between experiment and fitting is obtained by reducing the interlayer distance that had been calculated using the density functional theory for dimers. Further calculations using the highest level quantum mechanical calculations prove that the reduction in interlayer distance does not significantly increase the energy of the dimer. Cluster trimers adopt a dimer structure with the additional monomer slanted against the dimer, and tetramers take on a stacked structure. The structure evolution with cluster size is extraordinary, because from trimer to tetramer, one monomer needs to be rearranged, and neither the trimer nor the tetramer adopts the corresponding global minimum structure obtained using high level coupled-cluster theory calculations. This phenomenon may be related to the fast cooling process in superfluid helium droplets during cluster formation.

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

This perspective presents the current status and future directions in using electron diffraction to determine the structures of clusters formed in superfluid helium droplets. The details of the experimental setup and data treatment procedures are explained, and several examples are illustrated. The ease of forming atomic and molecular clusters has been recognized since the invention of superfluid helium droplet beams. To resolve atomic structures from clusters formed in droplets, substantial efforts have been devoted to minimizing the contribution of helium to diffraction signals. With active background subtraction, we have obtained structures from clusters containing a few to more than 10 monomers, with and without heavy atoms to assist with the diffraction intensity, for both neutral and ionic species. From fittings of the diffraction profiles using model structures, we have observed that some small clusters adopt the structures of the corresponding solid sample, even for dimers such as iodine and pyrene, while others require trimers or tetramers to reach the structural motif of bulk solids, and smaller clusters such as CS2 dimers adopt gas phase structures. Cationic clusters of argon clusters contain an Ar3+ core, while pyrene dimers demonstrate a change in the intermolecular distance, from 3.5 Å for neutral dimers to 3.0 Å for cations. Future improvements in reducing the background of helium, and in expanding the information content of electron diffraction such as detection of charge distributions, are also discussed.