  We measure the liquid argon scintillation response to electronic recoils in
the energy range of $2.82$ to $1274.6~{\rm keV}$ at null electric field. The
single-phase detector with a large optical coverage used in this measurement
yields $12.8 \pm 0.3 ~ (11.2 \pm 0.3)~{\rm photoelectron/keV}$ for
$511.0$-${\rm keV}$ $\gamma$-ray events based on a photomultiplier tube single
photoelectron response modeling with a Gaussian plus an additional exponential
term (with only a Gaussian term). It is exposed to a variety of calibration
sources such as $^{22}{\rm Na}$ and $^{241}{\rm Am}$ $\gamma$-ray emitters, and
a $^{252}{\rm Cf}$ fast neutron emitter that induces quasimonoenergetic
$\gamma$ rays through a $(n, n'\gamma)$ reaction with $^{19}{\rm F}$ in
polytetrafluoroethylene. In addition, the high light detection efficiency of
the detector enables identification of the $2.82$-${\rm keV}$ peak of
$^{37}{\rm Ar}$, a cosmogenic isotope in atmospheric argon. The observed light
yield and energy resolution of the detector are obtained by the full-absorption
peaks. We find up to approximately $25\%$ shift in the scintillation yield
across the energy range and $3\%$ of the energy resolution for the
$511.0$-${\rm keV}$ line. The Thomas-Imel box model with its constant parameter
$\varsigma=0.033 ^{+0.012} _{-0.008}$ is found to explain the result. For
liquid argon, this is the first measurement on the energy-dependent
scintillation yield down to a few ${\rm keV}$ at null field and provides
essential inputs for tuning the argon response model to be used for physics
experiments.
