Actinometry of O, N and F atoms

Abstract

The applicability of actinometry for measuring the absolute concentration of O, N and F atoms in discharge plasma was studied. For this purpose, concentrations of these atoms were measured downstream of an ICP plasma by means of the actinometry method and of appearance potential mass-spectrometry (APMS). Comparison of the results showed good agreement between the two methods. Since the excitation cross sections of electron states O(3p 3 P) and O(3p 5 P) applied in actinometry are well tested, this allows using the APMS method for absolute calibration of the theoretical excitation cross sections for N and F atoms. As a result, total excitation cross sections σ e total of the atomic levels N(3p 4 P o ), F(3p 2 P o ) and F(3p 4 D o ) have been obtained for the first time. Since different types of electron energy distribution function (EEDF) were observed (Maxwellian, bi-Maxwellian and Druyvesteyn) the influence of these possible EEDF types on actinometric coefficients C Ar X ( X = O, N, F), that link the ratio of the atom and actinometer intensities I X / I Ar with that of their concentrations [ X ]/[Ar], was also analyzed. It was shown that at the same ionization rate (effective electron temperature) the excitation rate constants k e X , Ar are highly sensitive to the shape of EEDF, whereas actinometric coefficients C Ar X depend on it only slightly. Dependence of actinometric coefficients on electron temperature C Ar X ( T e ) is positive if the emitting level of the X -atom is lower than that of the actinometer, and negative if vice versa. The energy difference between the emitting states of O and Ar atoms is maximal (~3 eV), so that C Ar O ( T e ) is not constant for a whole range of electron temperatures typical for discharge plasmas (~2–8 eV). For nitrogen atoms C Ar N ( T e ) varies considerably with T e only when T e < 4 eV. In the case of fluorine atoms the energy difference of emitting F and Ar states is only ~1 eV and coefficient C Ar F ( T e ) is nearly constant in a wide region of T e > 1.5 eV.