Two-dimensional spatially resolved excitation and rotational temperatures as well as electron number density measurements in capacitively coupled microwave plasmas using argon, nitrogen and air as working gases by spectroscopic methods

Abstract

Two-dimensional spatially resolved mappings of excitation temperatures, rotational temperatures and electron number densities have been made for capacitively coupled microwave plasmas using argon, nitrogen and air as working gases. The influence of additions of hydrogen to the working gases on the temperature and electron number density profiles in the case of the argon- and nitrogen-CMP is described and found to result in a decrease of the temperatures and the electron number densities in the plasma centre as well as in an increase of these values in the peripheral regions of the plasma. Organic compounds such as ethanol, which still do not influence the plasma stability of the air-CMP when added to the sample solutions at concentrations of up to 30% (v:v), were found to lead to an increase of the rotational temperature in both the plasma centre and the peripheral regions of the investigated air-plasma, whereas the excitation temperature and the electron number density profiles are not influenced. Not only the shape but also the investigated physical parameters of the different plasmas strongly change when different amounts of easily ionizable elements are present in the sample solutions. Already the addition of as few as 10 mmol l −1 of Li, and in particular of Cs, to the aqueous sample solutions results in an extreme change of the geometry of the plasma and in a decrease of the excitation temperatures and electron number densities of the Ar-, N 2- and air-CMPs from 5000 K and 10 14 to 4000 K and 10 12 level in the central region, respectively.