Assessing the role of asymmetric charge transfer reactions in analytical glow discharges in argon

Charge transfer reactions between argon ions and analyte atoms occur at a much higher rate in analytical glow discharges than in other similar plasmas, as follows from experimental Boltzmann plots of glow discharge emission spectra and the spectra generated by an inductively coupled plasma. A simple model of a Grimm-type discharge suggests that charge transfer-excited emission of the analyte elements comes largely from the cathode sheath, due to a high collision rate between the sputtered analyte atoms and argon ions accelerated in the cathode fall. The contribution to this radiation from the negative glow is several times lower. The mentioned enhancement of the charge transfer-excited emission in the cathode sheath is, however, not sufficiently high to explain the anomalously high rates of those charge transfer reactions relative to other plasmas. The different characteristics of emission from the cathode sheath and the negative glow also means that Boltzmann plots of glow discharge spectra cannot be interpreted in the conventional way. Further work is needed to explain the observed features of glow discharge excitation, as reflected in the emission spectra. [Display omitted] •Boltzmann plots of GDOES emission spectra are discussed.•Charge transfer excitation is anomalously strong in GDOES.•The rates of Ar+-Me0 charge transfer reactions are assessed.•ACT-excitation rates peak at the rim of the cathode sheath.•Excitation in negative glow resembles excitation in ICP.