Atomically resolved phase transition of fullerene cations solvated in helium droplets.

M Kuhn,M Renzler,M Simpson,Y Wang,A Lindinger,J Cami,F Martín,R Wester,P Scheier,J Postler,A Mauracher,M K Beyer,S Spieler,H Linnartz,M Alcamí,A G G M Tielens,S Ralser

doi:10.1038/ncomms13550

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 HenC60+ complexes with n exceeding 100 via electron ionization of helium nanodroplets doped with C60. 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 HenC60+ 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.

Highlights

Helium has a unique phase diagram and below 25 bar it does not form a solid even at the lowest temperatures
We show the appearance of distinct changes in the matrix shift reflecting phase transitions of the adsorbed helium from solid to liquid and from liquid to superfluid
The first 32 helium atoms occupy the sites above the centres of the hexagonal and pentagonal carbon rings

Summary

Introduction

Helium has a unique phase diagram and below 25 bar it does not form a solid even at the lowest temperatures. Fullerene cations, such as C6þ0 , provide powerful probes of the transition from adsorbant interaction with the host cation to mutual adsorbant interaction, as—in contrast to planar aromatic structures—the curved surface allows a fully covered commensurate helium monolayer[24,25] We use this aspect, in conjunction with the characteristic wavelength shift (typically around 0.02 nm for the first adsorbed He atom)[9] introduced into electronic transitions by the He-C6þ0 interaction, to follow the transition from the solid to the liquid phase as a function of the number of helium atoms adsorbed.

Methods

Results

Conclusion