Abstract
By direct spatial imaging and spectral analysis of recombination luminescence we have observed in silicon (1) the drift of photoproduced excitons from the crystal surface into a strain potential well; (2) ideal-gas behavior of the strain-confined excitons in the well between 25 and 5 K; (3) a real-space condensation near T=4 K which defines the first-order gas-liquid phase transition; and (4) luminescence from excitonic molecules.