An ideal, defect-free semiconductor quantum well structure is a useful  model system to illustrate the fundamental processes in typical PL experiments. The discussion is based on results published in Klingshirn (2012) and Balkan (1998).
The fictive model structure for this discussion has two confined quantized electronic and two hole subbands, e, e and h, h, respectively.
The linear absorption spectrum of such a structure shows the exciton resonances of the first (e1h1) and the second quantum well subbands (e, h), as well as the absorption from the corresponding continuum states and from the barrier.