Resonant Interactions between Localized LO Phonons and Continuum States of Donors in CdS and CdSe

The first longitudinal-optical- (LO-) phonon replica of the $I_2$ line (decay of an exciton bound to a neutral donor) in CdS and CdSe is structured and broadened. The replica is roughly Lorentzian in shape, shifted to lower energy than the nominal LO-phonon energy and has a sharp notch near the center of the line. These features are shown to arise from coupling between LO phonons and the donor electron. The line is lifetime broadened by ionization of the donor. Virtual excitations of the donor result in frequency shifts and cross coupling of different LO-phonon modes. The notch is an interference effect resulting from the coupling of the localized mode existing immediately after the electron and hole recombine with less-damped modes, located more distant from the donor.