The populations of hydrogen atoms in the excited states (In=2 and 3) across the potential step in a DC discharge at low pressure have been measured bylaser resonance scattering, with the laser tuned to the Balmer alpha transition (Hα, wavelength 656.3 nm). The results were used first to deduce the amplitude of the ion-acoustic turbulence, which was excited by the electron-ion streaming instability at the step. The amplitude was high enough to explain the anomalously high electrical resistivity there. Second, the results were used to discuss the cause of the luminosity jump, due to the behavior of electrons distributed in the potential of the ion-acoustic turbulence.
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