Diagnostics of collisions between electrons and water molecules in near-ultraviolet and visible wavelengths

Abstract

We studied 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 ${}^2{\Sigma }^{+}$ - X ${}^2\Pi$) band, and by the emission of ionic H${}_2$O${}^{+}$ (A ${}^2$A${}_1$ - X ${}^2$B${}_1$) and OH${}^{+}$ (A ${}^3\Pi$ - X ${}^3{\Sigma }^{-}$) band systems. Emission cross sections and reaction channel thresholds were determined for energies between 5 - 100 eV. We find that electron impact dissociation of H${}_2$O results in an emission spectrum of the OH (A ${}^2{\Sigma }^{+}$ - X ${}^2\Pi$) band that is distinctly different than 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.

9 Figures, 8 Tables