We report an investigation of white-light continuum generation during
self-focusing in extended transparent media using 140-fs Ti:sapphire l
aser pulses. A band-gap threshold is found above which the width of th
e continuum tends to increase with increasing band gap and below which
there is no continuum generation. This is, to our knowledge, the firs
t report of a parameter predicting the width of the continuum in conde
nsed media, Multiphoton excitation of electrons into the conduction ba
nd is proposed as the primary mechanism reponsible for the observation
s.