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

Abstract

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 2 Σ +  − X 2 Π) band, and by the emission of ionic H 2 O + (A 2 A 1  − X 2 B 1 ) and OH + (A 3 Π − X 3 Σ − ) 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 H 2 O results in an emission spectrum of the OH (A 2 Σ +  − X 2 Π) 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.