  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.
