Atomically resolved phase transition of fullerene cations solvated in helium droplets.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 27874002.
- Also identified by DOI 10.1038/ncomms13550 and PMC identifier 5121423.
- Licence recorded as CC BY.
- The licence permits redistribution, so the abstract is shown in full and the full text is available from the publisher.
Abstract
Helium has a unique phase diagram and below 25 bar it does not form a solid even at the lowest temperatures. Electrostriction leads to the formation of a solid layer of helium around charged impurities at much lower pressures in liquid and superfluid helium. These so-called 'Atkins snowballs' have been investigated for several simple ions. Here we form He<sub>n</sub>C<sub>60</sub><sup>+</sup> complexes with n exceeding 100 via electron ionization of helium nanodroplets doped with C<sub>60</sub>. Photofragmentation of these complexes is measured by merging a tunable narrow-bandwidth laser beam with the ions. A switch from red- to blueshift of the absorption frequency of He<sub>n</sub>C<sub>60</sub><sup>+</sup> on addition of He atoms at n=32 is associated with a phase transition in the attached helium layer from solid to partly liquid (melting of the Atkins snowball). Elaborate molecular dynamics simulations using a realistic force field and including quantum effects support this interpretation.