We have measured the electron optical capture cross section, σ0n(hν), of EL2 (the most important native center in GaAs) using a new technique which we have recently developed: optical admittance spectroscopy. This is a spectroscopic technique based on the measurement of the capacitance and conductance of a junction under monochromatic light of energy hν. This technique allows the measurement of the spectrum σ0n(hν) of each center located in the band gap. We have measured the electron photoionization cross section of the EL2 center, σ0n(hν), at three different temperatures within a range limited at high temperature by thermal emission and at low temperature by photoquenching (a feature characteristic of EL2 below 140 K). The study of the experimental data reveals that this center has a more complex nature than that of a simple defect. It seems to behave like a family of very close levels corresponding to similar atomic structures and located near the midgap. These results also reveal the existence of a sha...