Formation of Positively Charged Liquid Helium Clusters in Supercritical Helium and their Solidification upon Compression

Abstract

Positively charged ions were produced in supercritical helium at temperatures from 6 to 10 K and up to 2 MPa using a corona discharge. Their mobility was measured via currentvoltage curves and the hydrodynamic radius was derived using Stokes law. An initial increase and subsequent decrease of hydrodynamic radius was observed and interpreted in terms of growth, compression and solidification of ion clusters. The mobility was modelled using a van der Waals-type thermodynamic state equation for the ion-in-helium mixed system and a temperature-dependent Millikan-Cunningham factor, describing experimental data both in the Knudsen and the Stokes flow region. Regions of maximum hydrodynamic radius and large 1

Theoretical modeling of electron mobility in superfluid 4He

“…This model has been successful in describing electron mobility in supercritical and liquid helium, 40 helium gas at low temperature, 41 and positive ion mobility in liquid helium 42 and supercritical helium. 43 Although the model itself is less computationally demanding than DFT and not limited to a specific system, it obscures many of the microscopic details of ion solvation and the role of dissipative processes.…”

Atomically resolved phase transition of fullerene cations solvated in helium droplets

“…The formation of a solid layer of helium around an ionic impurity in bulk superfluid helium is often referred to as ‘Atkins snowball', and has been investigated both experimentally and theoretically for several simple ions1718192021. Gas-phase experiments of helium droplets containing an ion offer the opportunity to study the structure of these snowballs in detail.…”

Atomic fluorescence emitted from a corona discharge in helium above and below saturated vapour pressure

“…This is important because defects and impurities generally affect electronic structure and dynamics rather strongly. The exceptional purity of cryogenic helium has recently been exploited in studies of nucleation and growth of nanoparticles around ions in supercritical helium [8].…”

“…A variety of excitation methods have been reported, including radioactive particles [16][17][18][19][20][21], electron beams [1,[22][23][24][25][26][27][28][29], proton beams [30,31], discharges [8,[32][33][34][35][36][37] and field emission [38,39]. Helium clusters and droplets have been excited with electrons [40][41][42][43][44][45] and with VUV radiation [2,7,11,[46][47][48][49][50][51][52][53][54][55][56][57], including high harmonic generation [58][59][60].…”