Atomic and molecular spectra of normal liquid 4He excited by corona discharges

Liquid 4He at 4.2 K and different pressures up to 8 MPa is excited by positive and negative corona discharges. Emission of He I atomic lines and He2 molecular bands is observed. In negative corona, the line spectrum has distinct blue-shift and broadening, which becomes stronger with increasing pressure. The rotational structure of the molecular bands is resolved at pressures of 0.1–0.2 MPa. A blue shift of the Q-branch maximum with pressure is observed. A rotational temperature of 900 K is estimated from the d3Σu + − b3Πg molecular band emission. Positive corona was produced on a point anode with smaller electrode radius and higher voltages than for the negative corona. The electric currents for the negative and positive corona are similar. The radiation from the positive corona discharges has spectral features which differ qualitatively from the negative corona discharges. The spectra with a positive corona have marked asymmetries with greater intensities in the longer wavelength (red) wings.