Energetic ion, atom, and molecule reactions and excitation in low-current H₂ discharges: Spatial distributions of emissions

Spatial distributions of H_α, H_β, and the near-uv continuum emission from the H₂ a ³Σ_g⁺ state are measured and compared with a model for low-current electrical discharges in H₂ at high E/N and low Nd, where E is the spatially uniform electric field, N is the gas density, and d is the electrode separation. Data are analyzed for 300 Td<E/N<45 kTd, d=0.04 m, and 2×10²¹<N<2.6×10²² m⁻³. (1 Td=10⁻²¹ V m²) The excitation is produced by electrons and by hydrogen atoms and molecules with mean energies from 5 to 1500 eV. Electron-induced emission, dominant at low E/N and low pressures, is distinguished by its buildup toward the anode. Excitation of H_α by fast H atoms dominates at high E/N and increases toward the cathode. The observed H_α emission at low E/N is normalized to previous experiments to yield absolute experimental excitation coefficients for all E/N and Nd. Small adjustments of model parameters yield good agreement with H_α data. Cross sections are derived for excitation of the H₂ near-uv continuum by H atoms. Spatial and pressure dependencies of H_α and H₂ near-uv emissions agree well with a model in which reactions of H₂⁺, H₃⁺, and H⁺ ions with H₂ lead to fast H atoms and H₂ molecules, which then excite H atoms or H₂ molecules.