  We have closely examined the emission spectrum at the heavy-hole exciton
resonance in a high-quality GaAs multi-quantum well (MQW) sample using
picosecond excitation-correlation photoluminescence (ECPL) spectroscopy.
Dynamics of the ECPL signal at low and high energy sides of the excitonic
photoluminescence (PL) peak shows complementary behavior. The ECPL signal is
positive (negative) below (above) the PL peak and it changes sign within a
narrow band of energy lying between excitonic absorption and emission peaks.
The energy at which this sign change takes place is interpreted as the
excitonic mobility edge as it separates localized excitons in quantum dot-like
states from mobile excitons in quantum well-like states.
