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

TL;DR

This study investigates electron impact dissociation of water vapor across 200-850 nm wavelengths, analyzing emission spectra, cross sections, and reaction thresholds to distinguish different excitation mechanisms relevant to planetary astronomy.

Contribution

It provides new measurements of emission cross sections and reaction thresholds for water dissociation by electrons, highlighting spectral differences from other excitation mechanisms.

Findings

Emission spectrum dominated by Hydrogen Balmer series and OH bands

Distinct spectral signature of electron impact dissociation compared to other mechanisms

Potential for remote sensing of physical reactions in astrophysical environments

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.