Transition radiation and the origin of sonoluminescence

It has been shown by Liberati et al. [Phys. Rev. D 61, 85023 (2000)] that a dielectric medium with a time-dependent refractive index may produce photons. We point out that a free electric charge that 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.