Modeling of glow discharge optical emission spectrometry: Calculation of the argon atomic optical emission spectrum

The optical emission spectrum of the argon atomic lines in a glow discharge is calculated, using a collisional-radiative model for argon, which was recently developed (A. Bogaerts et al., Collisional-radiative model for an argon glow discharge, J. Appl. Phys., vol. 84, No 1, 1998). It is shown that the lines corresponding to 4p --> 4s transitions clearly dominate the spectrum. They are, however, not responsible for the characteristic visible light in the glow discharge, because they are lying between 700 and 1000 nm, which is mainly in the near infrared. The characteristic blue light of the glow discharge is caused by the lines corresponding to 5p --> 4s transitions (lying in the blue-violet part of the spectrum). Beside these two most important line groups (the so-called 'red' and 'blue' lines) a large number of other lines are present, making the entire argon spectrum quite complex. The calculated spectrum is compared with experimental spectra from the literature, and excellent qualitative agreement is obtained. The calculated spatial distributions of optical emission lines originating from low excited levels (i.e., 4p, 3d, 5s, 5p, 4d, 6s) show a maximum in the cathode glow, caused by fast argon ion and atom impact excitation, to these levels, and a second maximum in the beginning of the negative glow, due to electron impact excitation. The maximum in the cathode glow is very pronounced for lines originating from the 4p levels, which is in agreement with experimental observations. The higher excited levels are not populated by fast argon ion and atom impact excitation but only by electron impact excitation; hence, lines originating from these levels exhibit only a maximum in the beginning of the negative glow. (C) 1998 Elsevier Science B.V. All rights reserved.