Spatially resolved observation of a radio-frequency-powered glow discharge plasma for emission spectrometric analysis

In glow discharge optical emission spectrometry, two-dimensional emission images for iron atomic lines were measured by using an imaging spectrograph equipped with a CCD detector, when a radio-frequency (r.f.) power source was employed for excitation. Emission images at the Fe I 371.99-nm and the Fe I 375.82-nm lines, having different excitation energies, were analyzed by the two-line method to obtain the spatial distribution of the excitation temperature in the plasma. Their emission intensities had a concentric-circle-like distribution along the radial direction of the plasma to become weaker towards the surrounding portion, which was very similar to a direct-current (d.c.) glow discharge plasma. On the other hand, the spatial distribution in the excitation temperature became relatively uniform over the central portion of the plasma, also being analogous between the r.f. and the d.c. glow discharge plasmas. These results imply that there is a major excitation process that occurs in a glow discharge plasma regardless of the power modes.