Spatial distributions of ${\text{H}}_{\alpha }$, ${\text{H}}_{\beta }$, and the near-uv continuum emission from the ${\text{H}}_2$ $a{{}^3\Sigma }_g^{+}$ state are measured and compared with a model for low-current electrical discharges in ${\text{H}}_2$ 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 $300\text{Td}<E/N<45\text{kTd}$, $d=0.04\text{m}$, and $2\times {10}^{21}<N<2.6\times {10}^{22}{\text{m}}^{-3}$. $\left(1\text{Td}={10}^{-21}\text{V}{\text{m}}^2\right)$ 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 ${\text{H}}_{\alpha }$ by fast H atoms dominates at high $E/N$ and increases toward the cathode. The observed ${\text{H}}_{\alpha }$ 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 ${\text{H}}_{\alpha }$ data. Cross sections are derived for excitation of the ${\text{H}}_2$ near-uv continuum by H atoms. Spatial and pressure dependencies of ${\text{H}}_{\alpha }$ and ${\text{H}}_2$ near-uv emissions agree well with a model in which reactions of ${\text{H}}_2^{+}$, ${\text{H}}_3^{+}$, and ${\text{H}}^{+}$ ions with ${\text{H}}_2$ lead to fast H atoms and ${\text{H}}_2$ molecules, which then excite H atoms or ${\text{H}}_2$ molecules.

• Received 13 May 2009

DOI:https://doi.org/10.1103/PhysRevE.80.016408