  The XENON100 dark matter experiment uses liquid xenon in a time projection
chamber (TPC) to measure xenon nuclear recoils resulting from the scattering of
dark matter Weakly Interacting Massive Particles (WIMPs). In this paper, we
report the observation of single-electron charge signals which are not related
to WIMP interactions. These signals, which show the excellent sensitivity of
the detector to small charge signals, are explained as being due to the
photoionization of impurities in the liquid xenon and of the metal components
inside the TPC. They are used as a unique calibration source to characterize
the detector. We explain how we can infer crucial parameters for the XENON100
experiment: the secondary-scintillation gain, the extraction yield from the
liquid to the gas phase and the electron drift velocity.
