\begin{abstract}
We studied the effects of ionization in a sonoluminescing (SL) bubble
within the hydrodynamic framework.
The thermodynamic variables and the degrees of ionization
inside the bubble throughout an oscillation cycle
are obtained by solving the hydrodynamic equations assuming spherical symmetry.
Several models are used to compute the emitted radiation,
which are then compared with experimental data.
Numerical results show that shock waves are absent
in the stable SL regime, and compressional waves are already strong enough
to produce moderate temperature and ionization.
The degrees of ionization at the bubble center are found to be within
7\% to 30\%, and Ar$^+$ is the only dominant ion.
Moreover, an opacity-corrected blackbody radiation model
gives the peak power, pulse widths and spectra
that agree very well with the experimental data.
\end{abstract}
\pacs{PACS numbers: 78.60.Mq, 47.40.-x, 52.50.Lp, 43.25.+y}