Diagnostics of Collisions between Electrons and Water Molecules in Near-ultraviolet and Visible Wavelengths

We studied the dissociation reactions of electron impact on water vapor for several fragment species at optical and near-ultraviolet wavelengths (200-850 nm). The resulting spectrum is dominated by the hydrogen Balmer series, by the OH (A ²Σ⁺ - X ²Π) band, and by the emission of ionic H₂O⁺(A ²A₁ - X ²B₁) and OH⁺(A ³Π - X ³Σ^-) band systems. Emission cross sections and reaction channel thresholds were determined for energies between 5 and 100 eV. We find that the electron impact dissociation of H₂O results in an emission spectrum of the OH (A ²Σ⁺ - X ²Π) band that is distinctly different from the emission spectra from other excitation mechanisms seen in planetary astronomy. We attribute the change to a strongly non-thermal population of rotational states seen in planetary astronomy. This difference can be utilized for remote probing of the contribution of different physical reactions in astrophysical environments.