New measurements of the energy dependence of the photoionization cross section of the DX center in Si-doped AlxGa1-xAs are reported. With the use of a tunable infrared laser which provides sufficient light intensity in a very narrow wavelength range, the photoionization cross section has been measured over 68 orders of magnitude in samples with a variety of alloy compositions and doping concentrations. No measurable signal was observed for photon energies less than 0.8 eV. Data are analyzed in terms of a simple model which gives a value of 1.41.8 eV for the photoionization threshold energy. This energy is much larger than the DX-center binding energy, confirming that there is a large relaxation of the lattice when charge is captured at the DX center. The phonon mode involved in the lattice relaxation is found to be 5.5 meV. © 1988 The American Physical Society.