  This Letter details a measurement of the ionization yield ($Q_y$) of 6.7 keV
$^{40}Ar$ atoms stopping in a liquid argon detector. The $Q_y$ of 3.6-6.3
detected $e^{-}/\mbox{keV}$, for applied electric fields in the range 240--2130
V/cm, is encouraging for the use of this detector medium to search for the
signals from hypothetical dark matter particle interactions and from coherent
elastic neutrino nucleus scattering. A significant dependence of $Q_y$ on the
applied electric field is observed and explained in the context of ion
recombination.
