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
The liquid 4 He at fixed temperature 4.2 K and different pressures up to 8 MPa was excited by corona discharge of both negative and positive polarity. Emission of He I atomic lines and He 2 molecular bands are observed. In negative corona the lines spectra show a distinct blue-shift and line-broadening, which becomes stronger with the pressure increasing. The rotational structure of molecular bands is resolved at pressures (0.1-0.2) MPa. The blue shift of the Q-branch maximum at different pressures was observed. Rotational temperature of 900 K has been estimated for the 3 3-u g d b + Σ Π molecular band. A positive corona was realized on a point anode for fewer radii of the electrode and larger voltage than in the negative corona. Electric currents in both negative and positive corona differ weakly. Spectral analysis of the radiation from the positive corona shows qualitative differences of spectral features of these discharges. The spectra observed in the positive corona have marked non-symmetric shape. The asymmetric atomic and molecular spectra show an increased intensity of their long-length (red) wings. PACS: 36.40.Mr Spectroscopy and geometrical structure of clusters; 71.35.Aa Frenkel excitons and self-trapped excitons; 73.20.-r Electron states at surfaces and interfaces; 78.40.-q Absorption and reflection spectra: visible and ultraviolet.
The mobility of positively charged ions in supercritical helium at temperatures from 6 to 11 K and up to 20 bar was measured using corona discharges. Regions of increased and decreased hydrodynamic radius are interpreted in terms of growth, compression and solidification of ion clusters. Modelling of the mobility using a thermodynamic state equation for the ion helium mixed system and a temperature dependent Cunningham correction provides best match with experimental data for a van der Waals helium-ion interaction term. Regions of large hydrodynamic radius and large compressibility are attributed to liquid-like behaviour.
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