Some observations of an anomalous excitation behavior of chromium emission lines in an argon radio-frequency powered glow discharge plasma

The emission characteristics of some chromium atomic lines in an radio-frequency powered glow discharge plasma (r.f.-GDP) are investigated by means of the excitation temperature estimated from their Boltzmann plots. It was found that such behavior depends on the drive frequency as well as the pressure of argon. Departures of the Boltzmann plots from a linear relationship are observed when the driving frequency exceeds 10 MHz, whereas the plots are well fitted to a straight line at driving frequencies of 3.56 and 6.78 MHz. Similarly, the Boltzmann plot deviates from a linear relationship when the argon pressure is reduced. The experimental observations show that the effect is mainly due to the fact that the intensities of the Cr I lines having excitation energies in the range of 4.1–4.2 eV are enhanced and therefore do not follow a linear regression expected from those of the other Cr I lines. It is therefore speculated that, in addition to thermodynamic excitation processes through electron collision, some additional excitation channels could exist under particular discharge conditions in the r.f.-GDP pointing to a potentially selective excitation of the 4.1-eV chromium lines.