Although the large lattice relaxation model (LLR) for electron capture at the donor related DX center in AlxGa1-xAs has seen wide acceptance over the last 12 years, there have been some recent proposals which have attempted to explain the experimental data with models that only require small lattice relaxation (SLR). One key piece of evidence supporting LLR is the large observed difference (in the case of Si-doped AlxGa1-xAs) between the optical (∼1.4 eV) and thermal (∼0.2 eV) ionization energies. The SLR model proposed that the lowest energy optical ionization was a very weak process, and that the optical transition which had been observed previously is a transition to a higher band. These arguments were supported by photoconductivity data showing a finite photo-ionization rate at energies as low as 200 meV. To resolve this question we have measured the photo-ionization cross section over 7 to 8 orders of magnitude using a tunable infrared laser as a source. A consistent optical ionization energy of about 1.4 eV was observed for 4 samples of differing alloy compositions and doping levels. In no case was there any detectable photo- ionization below 0.8 eV. A detailed discussion of these experiments examines the difficulty in obtaining such a large dynamic range optical spectrum. Of particular relevance are the issues of ionization detection, and the brightness of purity of the optical source. A thorough review of these issues and their impact on previous studies of the DX center is presented. © 1991 AIME.