We present examples of unusual “stairstep” differential flux spectra observed by the Apollo 14 Superthermal Ion Detector Experiment (SIDE) on the lunar dayside surface in Earth’s magnetotail. These spectra exhibit a relatively constant differential flux below some cut-off energy and then drop off precipitously, by about an order of magnitude or more, at higher energies. We propose that these spectra result from photoions accelerated on the lunar dayside by nonmonotonic potentials, potentials that do not decay to zero monotonically, and present a model for the expected differential flux. The energy of the cut-off and the magnitude of the differential flux are related to the properties of the local space environment and are consistent with the observed flux spectra. If this interpretation is correct, these surface-based ion observations provide a unique perspective that both complements and enhances the conclusions obtained by remote-sensed orbiter observations on the Moon’s exospheric and electrostatic properties.