Abstract
Flowing plasma jets are increasingly investigated and used for surface treatments, including biological matter, and as soft ionization sources for mass spectrometry. They have the characteristic capability to transport energy from the plasma excitation region to the flowing afterglow, and therefore to a distant application surface, in a controlled manner. The ability to transport and deposit energy into a specimen is related to the actual energy transport mechanism. In case of a flowing helium plasma, the energy in the flowing afterglow may be carried by metastable helium atoms and long-lived helium dimer ions. In this work a systematic investigation of the optical and spectroscopic characteristics of a supersonic flowing helium plasma in vacuum and its afterglow as function of the helium gas density is presented. The experimental data are compared with numerical modeling of the plasma excitation and helium dimer ion formation supported by a Computational Fluid Dynamic simulation of the helium jet. The results indicate that the plasma afterglow is effectively due to helium dimer ions recombination via a three-body reaction.
Highlights
Flowing plasma jets are increasingly investigated and used for surface treatments, including biological matter, and as soft ionization sources for mass spectrometry
One mechanism is related to the high excited state energy of metastable helium atoms which are formed during the plasma excitation, eventually from cascade transitions from higher excited states
Several investigations have indicated that associative ionization is a nearly resonant reaction with the 3d energy level, resulting in molecular ions in a vibrational excited state (ν = 4)[25]
Summary
Flowing plasma jets are increasingly investigated and used for surface treatments, including biological matter, and as soft ionization sources for mass spectrometry They have the characteristic capability to transport energy from the plasma excitation region to the flowing afterglow, and to a distant application surface, in a controlled manner. For a flowing helium plasma the dynamics of the energy transport in the afterglow is very complicated due to the peculiar helium plasma characteristics: presence of highly energetic metastable states and energetic excited states close to the ionization limit, as well as the prominent tendency, at increasing particle density, to form helium dimer ions[11,12]. The helium metastable atoms, Hem, have long lifetime and in principle, can live in the afterglow, giving rise eventually to nitrogen ion formation in air through de-excitation and penning ionization in air[11], Hem + N2 → N2+ + He + e− + ∆E1
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