A detailed defect energy level map was investigated for heterostructures of 26 unit cells of LaAlO3 on SrTiO3 prepared at a low oxygen partial pressure of 10−6 mbar. The origin is attributed to the presence of dominating oxygen defects in SrTiO3 substrate. Using femtosecond laser spectroscopy, the transient absorption and relaxation times for various transitions were determined. An ultrafast relaxation process of 2–3 ps from the conduction band to the closest defect level and a slower process of 70–92 ps from conduction band to intraband defect level were observed. The results are discussed on the basis of the proposed defect-band diagram.