  The response of liquid xenon to low-energy electronic recoils is relevant in
the search for dark-matter candidates which interact predominantly with atomic
electrons in the medium, such as axions or axion-like particles, as opposed to
weakly interacting massive particles which are predicted to scatter with atomic
nuclei. Recently, liquid-xenon scintillation light has been observed from
electronic recoils down to 2.1 keV, but without applied electric fields that
are used in most xenon dark matter searches. Applied electric fields can reduce
the scintillation yield by hindering the electron-ion recombination process
that produces most of the scintillation photons. We present new results of
liquid xenon's scintillation emission in response to electronic recoils as low
as 1.5 keV, with and without an applied electric field. At zero field, a
reduced scintillation output per unit deposited energy is observed below 10
keV, dropping to nearly 40% of its value at higher energies. With an applied
electric field of 450 V/cm, we observe a reduction of the scintillation output
to about 75% relative to the value at zero field. We see no significant energy
dependence of this value between 1.5 keV and 7.8 keV. With these results, we
estimate the electronic-recoil energy thresholds of ZEPLIN-III, XENON10,
XENON100, and XMASS to be 2.8 keV, 2.5 keV, 2.3 keV, and 1.1 keV, respectively,
validating their excellent sensitivity to low-energy electronic recoils.
