Exciton Excitation Spectra in CdS

Optical excitation spectra of exciton emission lines in CdS are presented. It is shown that the center responsible for the ${\mathrm{I}}_1$ emission, an exciton bound to a neutral acceptor, is created by the formation of free excitons which are subsequently trapped or bound to the impurity, or by the direct formation of bound excitons on the impurity site. The centers responsible for an $I_5$ emission line at 4869.5 Å and for an $I_2$ emission line at 4868.3 Å are also created by the formation of free excitons which are subsequently bound to a localized site, but more importantly they are created by direct LO phonon assisted formation of excitons bound to neutral and ionized impurities. It is concluded that the impurity is a donor. Evidence of thermal dissociation of these complexes is also presented.