Atmospheric Helium Capillary Dielectric Barrier Discharge for Soft Ionization: Determination of Atom Number Densities in the Lowest Excited and Metastable States

The populations of the lowest excited helium states 2s ³S₁, 2s ¹S, 2p ³P⁰_J, and 2p ¹P⁰ created in an atmospheric helium capillary dielectric barrier discharge were determined by means of optical emission spectroscopy. The emitted intensities of 388, 501, 587, and 667 nm lines were measured side-on and end-on with respect to the discharge axis. The comparison of optically thin side-on spectra with end-on spectra, which exhibited the absorption effects in the line kernels, enabled the determination of the average values of the number densities n₁ in the considered He states along the plasma length L. The field of the theoretical profiles for a series of the n₁L parameters pertinent to the experimental conditions was calculated for each line. By introducing the experimental data into the field of calculated curves, n₁L corresponding to the particular state could be obtained. The measurements of the emission profiles were done as a function of the discharge voltage in the range covering homogeneous as well as filamentary DBD operation mode. Due to nonuniformity of the excited atom density distribution along the plasma, the values of n₁ could be obtained only in the homogeneous operation mode where the nonuniformity was small. The following maximum values were found for the number densities in the investigated states: n₁^av (2s ³S₁) = (2.9 ± 1.1) × 10¹³ cm^–3, n₁^av (2s ¹S) = (1.4 ± 0.5) × 10¹³ cm^–3, n₁^av (2p ³P⁰_J) = (1.1 ± 0.4) × 10¹³ cm^–3, n₁^av (2p ¹P⁰) = (4.2 ± 1.6) × 10¹² cm^–3, and they represent the average populations along the plasma column in the capillary.