  The decay of the optical phonon into the two phonons of smaller energy is
calculated for Si nanocrystals. The rate of the process is in the range of 1 to
10 ps. Such anharmonic phonon decay may control the energy relaxation rate of
excited carriers in Si/SiO$_2$ nanocrystals. Relevance of the phonon decay to
the experimentally observed hot carrier photoluminescence is discussed.
