A model in which we interpret the observed temperature dependence of the relative intensities of symmetryallowed phonon-assisted exciton emission in Si and Ge is developed. The model is based on a splitting of the exciton ground state combined with different transition rates for the split exciton levels. Symmetry arguments are applied to reduce the relative transition rates for the various phonon-assisted transitions to a minimum number of parameters involving products of reduced matrix elements and energy denominators. If the parameters are determined by comparison with experiment, good quantitative agreement is achieved.

• Received 22 December 1975

DOI:https://doi.org/10.1103/PhysRevB.14.2448