The introduction of a d.c. bias current to an r.f. glow-discharge plasma led to an intensity increase of particular emission lines. It was considered that the excitation mechanism was investigated based on Boltzmann plots of chromium emission lines with and without the introduction of a bias current. In the case of Cr I lines, the Boltzmann plot follows a straight lines without bias-current introduction, whereas it bends upwards when a bias current is introduced. This effect could be explained from the fact that the excited levels having energies of 4.2 eV and 5.8 eV were more populated compared to the excited levels of 3.3 eV through collisions with introduced electrons. On the other hand, the Boltzmann plot of Cr II lines bends downwards with no bias currents, which is caused by the selective excitation of Cr ion species having excitation energies of 8 eV through charge-transfer collisions between Cr atoms and Ar ions. When bias currents were introduced, the charge transfer collisions could be less effective, leading to a straight-line relationship of the Boltzmann plot of the Cr II lines. These results effects suggest that introducing a bias current is effective to enhance the intensities of not only the Ni or Cr atomic emission lines, but also the other atomic-emission lines.
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