Investigation of atomic and molecular fluorescence spectroscopy of helium in different pressures and temperatures using a corona discharge

Fluorescence spectroscopy is a powerful tool for obtaining information on microscopic processes in discharge (corona) plasmas in dense media, such as high pressure supercritical gases and even liquids. Spectroscopic observations of the light emitted from an ionization zone near a tip electrode can be used to determine structural information of the local environment of the emitting atoms or molecules. The spectra observable are sensitive to the immediate surroundings of the emitting species, making them useful for the study of cold non-equilibrium plasmas at varying pressures and temperatures. The aim of this project is to further our understanding of helium (4He) in different phases, providing information to help understand the nature of the interaction between the helium atoms within this environment. In this work, spectra have been recorded for a corona discharge in gaseous and liquid helium. The experimental conditions covered a wide region of thermodynamic states in two different designs. The first experiment was carried out at cryogenic temperatures at 3.8, 4.0, 4.5 and 5.0 K under a range of pressure (0.1-5.0 bar). The second set of experiments were carried out at room temperature with range of pressures 0.1-30 bar. Several thousand spectra were recorded in the visible and near-infrared regions. Analyses were conducted on spectra including the atomic 3s 1S→2p 1P and 3s 3S→2p 3P transitions. These transitions showed line shifts, spectral broadening and intensity changes that were dependent on the magnitude of the pressure and temperature, and therefore on the thermodynamic phase. In addition to atomic lines, rotationally-resolved molecular emission bands arising from the 𝑑3Σ𝑢+→𝑏3Π𝑔 and 𝐷1Σ𝑢+→𝐵Π𝑔 transitions of the electrically excited helium dimer (excimer), He2∗, were recorded. Remarkably, and similarly to the analysis of atomic lines, the molecular (excimer) emission showed sharp and distinct lines at low temperature in the region below the saturated vapor pressure (SVP) of helium. However, lineshifts, linewidths and line intensities of the excimers increased strongly upon an increase in pressure, indicating that solvated He2∗ in the liquid phase exhibits hindered rotation. The rotation of excimers within the liquid phase was attributed to the location of the excimer within a large bubble. A further interesting observation was the abrupt changes of lineshift and linewidths with pressures that occurred by crossing the SVP curve for both atomic and molecular transitions. A model was developed to explain this in which clusters form around excimers and excited helium atoms before the helium liquefies. In a separate application of the corona discharge, it was employed to make plasmas from a mixture of pure air and perfume. The idea here was collecting spectra of this mixture with and without helium to explore and understand some of the emitted species generated in this mixture of plasmas at atmospheric pressure and to the first time.