Comparative investigation of the emission characteristics between 6.78-MHz radio-frequency and direct-current glow discharge plasmas for atomic emission spectrometry

Abstract

This paper focused on the characteristics of emission spectra excited by a Grimm-style glow discharge plasma source in a 6.78-MHz radio-frequency (RF) power mode, in comparison to a conventional direct-current (DC) power mode. For this purpose, Boltzmann plots for atomic emission lines of iron were analyzed in detail, when the RF forward power and argon pressure in the discharge chamber were selected as the experimental parameters. The Boltzmann plot for low-lying excited levels of iron atom (3.4–4.8 eV) was on a linear relationship to provide an excitation temperature, whereas emission lines having larger excitation energies (more than 5 eV) generally deviated upwards from the linear relation. Any systematic dependence on the spin multiplicity was not found among the high-lying excited levels in the Boltzmann plot. The reason for the phenomenon was considered as follows: (1) the excitation for the low-lying emission lines would be caused by a dominant thermal collision process and (2) any non-thermal process also contributed to excitations for the high-lying emission lines to cause their over-population as a whole. The resultant excitation temperature was obtained in a narrow range of 3400–3600 K, which was hardly affected by the discharge power and the argon pressure, and further, the excitation temperature was very similar to that of a DC plasma that we had already reported. This result suggested that the glow discharge plasma could be supported by a self-stabilized negative glow region, where the kinetic energy distribution of the particles would be changed to a lesser extent as far as a stable discharge could be maintained.