It has been shown by Liberati et al. [quant-ph/9904013] that a dielectric medium with a time-dependent refractive index may produce photons. We point out that a free electric charge which interacts with such a medium will emit quantum mechanically modified transition radiation in which an arbitrary odd number of photons will be present. Excited atomic electrons will also exhibit a similarly modified emission spectrum. This effect may be directly observable in connection with sonoluminescence.

13 pages. Discussion improved and references added. To appear in Phys. Rev. E