We calculate the quasiparticle density of states in a phonon plus higher-energy exciton model. Without explicitly including the excitons in the process, the data are inverted to recover an effective phonon spectral density which is found to be very nearly the input distribution divided by ($1+{\lambda }_{\mathrm{ex}}$) where ${\lambda }_{\mathrm{ex}}$ is the exciton contribution to the mass renormalization. A negative effective Coulomb "repulsion" is also obtained and the effective microscopic parameters do not reproduce exactly the initial density of states. Finally, the size of the structure in the region of the effective distribution is found to be enhanced by the presence of the exciton peak.

• Received 14 October 1988

DOI:https://doi.org/10.1103/PhysRevB.39.2726