Dissociative recombination and its impact on the line profile of the hydrogen Balmer series

Abstract

In low pressure low temperature plasmas (p ∼ Pa, Te ∼ eV) the upper levels of the Balmer series of the hydrogen atom are populated by several excitation channels. While electron impact excitation is dominant for so-called ionizing plasmas (Te → 10 eV), dissociative recombination (DR) via the molecular ion becomes dominant in so-called recombining plasmas (Te → ≲ 1 eV). In the intermediate range, several other channels may be relevant as well. The expansion region of an ECR discharge, which represents such a plasma, is analyzed regarding the composition of the excitation processes as well as the line profiles of the Balmer series (for Hα to Hδ). Significantly broadened line profiles compared to Doppler broadening of thermal hydrogen atoms are revealed. The resulting line profiles resemble two Gaussian components with distinct FWHM (narrow component close to apparatus profile: ≈25–30 pm; broad component: ≈50–100 pm), where the broad part contributes to 10%–40% to the emissivity, depending on the individual Balmer line and the plasma conditions. Collisional radiative modeling was applied in order to determine the contributions of the different excitation channels. It turned out that for each of the lines of the Balmer series, population via DR contributes to the total excitation with a similar share as the share of the broad component of the line profile. Hence, it is concluded that DR is responsible for the broad component in the Balmer line profiles. The implication of this analysis on the spectroscopic determination of the atomic hydrogen density using a simple line ratio method (Hγ/Fulcher) is illustrated.