The dephasing of excitonic localized states is investigated in ${\mathrm{CdS}}_{\mathit{x}}$${\mathrm{Se}}_{1\mathrm{-}\mathit{x}}$ mixed crystals at 2 K with low intensity femtosecond optical pulses. We observe a fast relaxation process which is first reported here. It is shown to be related to the creation of localized biexcitons via the exciton-biexciton transitions which act as an additional dephasing mechanism with a time constant ${\mathit{T}}_2^{\mathrm{ex}\mathrm{-}\mathrm{bi}}$ of 700 fs. The coherence between excitonic and biexcitonic states, which manifests by 800 fs period beats, is studied in detail. A simple model is presented which describes both the fast initial decay and the beating behavior.

• Received 1 March 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.1820