We show that light emission from different systems of silicon nanocrystals does behave as expected for indirect-band-gap quantum dots. Photoluminescence excited on the low energy part of the distribution of Si nanocrystals exhibits a set of narrow peaks associated with Si TA and TO momentumconserving phonon-assisted optical transitions. These spectra allow us to determine the ratio of nophonon transitions to TA and TO phonon-assisted processes over a wide range of confinement energies. The ratio between these recombination channels changes by 2 orders of magnitude with increasing confinement energy. For confinement energies above 0.7 eV the radiative transitions are governed by no-phonon quasidirect processes. [S0031-9007 (98)07199-3] PACS numbers: 78.55.Ap, 78.66.Li

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