Angew.Chem.Int.Ed.http://doi.org/f2v659(2014)

Credit: © MOLEKUUL.BE/ALAMY

The behaviour of atoms and molecules on small length-scales is intriguing, and whether salt clusters adopt the same face-centred-cubic crystalline structure as in the extended solid (pictured) is one problem that has piqued the interest of many chemists. Previous studies carried out in the gas phase — in which alkali-metal halide clusters were formed for example by sputtering or laser ablation — have suggested that they do. Now, a team led by Paul Scheier and Andrew Ellis from Universität Innsbruck and the University of Leicester have turned to liquid helium droplets to further probe the assembly of salt nanocrystals. The researchers reacted sodium and sulfur hexafluoride (SF6) within the droplets and found that face-centred-cubic structures do form, with particular stability conferred to clusters that consist of complete unit cells.

Sodium and SF6 dopants were introduced separately into the liquid helium droplets and chemical reactions were subsequently induced by electron impact. A wide range of cationic and anionic salt clusters were produced, with (NaF)nNa+ and (NaF)nF− among the main species. Consistent with previous studies in the gas phase, mass spectrometry analysis showed that energetically favourable structures are obtained for 'magic numbers' including n = 7, 22 and 37, which coincide with complete unit cells of the face-centred-cubic lattice. Other abundant species that incorporated sulfur were also observed, such as (NaF)n(Na2S)mNa+ and (NaF)nS−.

The researchers propose that clusters of sodium and clusters of SF6 are both first embedded within a droplet and only react when energy is provided through electron impact. The self-assembly of cationic clusters is thought to be initiated through two mechanisms: collision of a dopant with metastable excited helium atoms (He*) or charge transfer from molecular helium cations (He+) that form on electron impact. The formation of the anionic clusters occurs through electron transfer from a more exotic species — a metastable excited helium anion (He−) that has only recently been recognized as produced by electron impact on liquid helium droplets.