Electron-collision Helium Ion State Enhancement from Electron Beam for He II Two-photon Absorption Laser Induced Fluorescence

Species-specific investigation of particles using laser spectroscopic techniques rely on sufficient state population and transition wavelengths that are suitable for available lasers, vacuum components, and detection electronics. Commonly, laser-based fluorescence diagnostics target metastable or ground states to ensure ample initial state density. However, the transitions are often very energetic. Two-photon absorption laser induced fluorescence (TALIF) is ideal for probing energetic transitions from metastable and ground states in many species, but, some species, such as helium, are difficult to investigate with this technique since ground state He ion transitions are too energetic to be probed with TALIF laser wavelengths (~30 nm) and low-lying energy state metastables are rarely naturally populated in cold plasmas. Here we populate the 2s metastable state of singly ionized helium directly by injecting an energetic electron beam into a background helium plasma. The 2s state is then pumped to the 6s state by two  205 nm photons and fluorescence intensity from the 6s to the 4p decay is collected at  ~656 nm. We present, for the first time, He II velocity distribution functions (IVDF) measured with TALIF in an electron beam-energized helicon plasma. Helium ion temperature and bulk flow are determined from the IVDFs.