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

Spatial distributions of Hα, Hβ, and the near-uv continuum emission from the H2 a 3Σg+ 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 2x1021<N<2.6x1022 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 Hα by fast H atoms dominates at high E/N and increases toward the cathode. The observed Hα 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 Hα data. Cross sections are derived for excitation of the H2 near-uv continuum by H atoms. Spatial and pressure dependencies of Hα 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.