We have observed light emission from Ar atoms [Ar(I)], Ge atoms [Ge(I)]/clusters[Ge2], and Ge ions [Ge(II)] during the evaporation process of Ge in an Ar-gas atmosphere of a few Torr. The Ar(I) lines are observed regardless of whether the crucible is empty or filled with Ge. Ar atoms are excited through discharge initiated by electrons thermionically emitted from the hot W wire heating the crucible and accelerated by the potential applied to the wire. Since the emission due to Ge is observed only when the Ar(I) lines are observed, it is concluded that Ge atoms are excited by excited Ar atoms and possibly ions. Intensities of Ar(I) lines decrease by transferring energy to Ge atoms/clusters if the density of Ge vapor is high. Emission intensity diminishes with distance from the crucible surrounded by a heat reflector and a water-cooled shroud, dropping most rapidly for Ge(II), and progressively less rapidly for Ge(I), Ar(I), and Ge2. The Ge(II) lines are stronger for lower Ar pressures and for lower Ge vapor density, while the Ge2 line is stronger for higher Ar pressures. The relative intensities of Ar(I), Ge(I), GE(II), and Ge2 also depend on the structure of the evaporation source. Reabsorption of emitted light by Ge clusters modifies the profile of the emission spectrum at ~3.7 and 4.3 eV.
Streamer propagation in a non-uniform electric field under lightning impulse in short gaps insulated with natural ester and mineral oil