  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.
