Spectroscopic investigation of liquid helium excited by a corona discharge: Evidence for bubbles and "red satellites"

The establishment of corona discharges close to a point electrode under both negative and positive high voltage in normal liquid helium (LHe) at 4.2 K is reported. The experiments were carried out at constant temperature and pressures ranging from 0.1-10 MPa. Visible luminescence emitted from the zone close to the tip revealed lines due to excited He atoms and molecules. The molecular luminescence showed hot band emissions with vibrational levels populated up to v = 2. Rotational temperatures of 800 K were estimated showing that the excitations do not thermalise. With increasing pressure the lines shifted to shorter wavelengths and became broader. The magnitude of the increase in width deviated from what is expected from the gas phase and from classical line broadening theory and rather showed similarities to the behavior of bubbles in LHe. The detailed analysis of the rotational line intensity distribution revealed the presence of an additional radiator at the long wavelength side of molecular bands that we tentatively assign to "red satellite" emission. For corona discharges with positive tip polarities both atomic and molecular lines showed "red satellite" bands with much larger intensity than for negative polarity. The origin of the red satellite and the polarity dependence is unclear yet.