Abstract
The radiative lifetime of free excitons in GaAs is unambiguously determined for the first time using ultrapure material grown by molecular-beam epitaxy. Its value amounts to 3.3\ifmmode\pm\else\textpm\fi{}0.5 ns in the limit of very low temperatures. Consequently, the oscillator strength of free excitons is on the order of unity, in contrast to what commonly is deduced from the total absorption cross section. At excitation densities above 0.5 W/${\mathrm{cm}}^{2}$ we have observed the radiative recombination of excitonic molecules (biexcitons) at 1.5146 eV in accordance with theoretical considerations.
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