  We report on the observation and numerical calculation of ionization spectra
of highly Stark-shifted Rydberg states of rubidium beyond the classical
ionization threshold. In the numerical calculations, a complex absorbing
potential (CAP) allows us to predict the energy levels and ionization rates of
Rydberg states in this regime. Our approach of adjusting the CAP to the
external electric field reduces the number of free parameters from one per
resonance to a single one. Furthermore, we have measured the ionization spectra
of magneto-optically trapped rubidium atoms which are excited to principal
quantum numbers of 43 and 70 at various electric fields. The emerging ions are
detected using an ion optics. We find good agreement between the numerically
and experimentally obtained spectra.
