The influence of the spatial nonuniformity on the measurement of Ar*(1s5) density by the self-absorption technique

The self-absorption technique is a simple method to determine the line integrated density of metastable atoms in low-pressure plasmas. In order to employ this technique, it is necessary to make an assumption on the spatial profiles of both the emission intensity and the absorbing species, which usually are unknown and can be highly nonuniform. Taking Ar*(1s5) atoms in a capacitively coupled plasma as an example, the influence of nonuniformity on the measurement of the line integrated density of the absorbing species is investigated analytically. It is proved that when the two spatial profiles are the same, the obtained line integrated density is independent of the functional form of this profile. This is also true if the density of the absorbing species is small (weak absorption), even though the emission profile is different from that of the absorbing species. However, if the density is high (strong absorption), the choice of the profiles has a significant influence on the deduced line integrated density. In the experiment, it is found that in argon-oxygen mixture discharges, in which Ar*(1s5) density is low (weak absorption), the measured densities by self-absorption are in very good agreement with the results obtained from laser absorption. However, in pure argon discharge, when the density is high (strong absorption), the measured densities by self-absorption are significantly smaller than that by laser absorption. Both phenomena have been predicted by the model results. The smaller densities obtained by self-absorption in the pure argon discharge are attributed to the assumption of the same spatial profile used in the model.