As the sensitivity of gravitational-wave detectors increases, new sources of noise appear. A potential source of noise may arise from charge accumulating on the mirrors of the experiment, the origin of which can be related to UV photons from the surroundings. In order to test one hypothesis, we measured the photon emission spectrum from a type of ion pump that is used in the experiment, an Agilent VacIon Plus 2500 l/s. We found that there is significant emission of UV photons above 5 eV, capable of knocking electrons off mirrors or surrounding surfaces and charging them. Photon emission measurements were taken as a function of gas pressure, ion-pump voltage setting, and type of pumped gas. The overall emission and shape of the measured photon spectrum are consistent with bremsstrahlung as the mechanism for the production of the photons.