Shape of atomic lines emitted by cryoplasma in helium

Abstract

Experimental data on spectral shape of the line 706nm emitted by corona discharge in liquid and gaseous cryogenic helium are presented. The data have their explanation in a frame work of the bubble model. The long-range interaction between excited He* atoms and He atoms in the ground state was used for a description of the fluorescence measurements. The 'hump' like repulsive part of the inter-atomic potential at intermediate internuclear separations around 5 Å was calculated by using the full configuration interaction method as implemented in the MOLPRO code. The repulsion gives rise to the establishment of cavities around excited atoms in liquid helium and radius and profiles of the cavity boundaries were calculated for this potential and different pressures. The potential was used to simulate the atomic line fluorescence profile as a function of external pressure between 0.1 and 3.5 MPa. The fluorescence shows characteristic line shifts and widths as a function of pressure. By comparison with the theoretical predictions, we found evidence that for corona discharge-excited liquid helium the He* atoms reside in a cavity and hence emit from within the liquid phase. The measurements carried out at higher temperature 5.1K give spectra are characterised for the bubble states similar to the states were obtained in superfluid He II at 1.8 K.