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 $(n, ngamma)$ 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.
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