Three-Pulse Femtosecond Spectroscopy of PbSe Nanocrystals: 1S Bleach Non linearity and Sub-Band-Edge Excited-State Absorption Assignment

Above band-edge photoexcitation of PbSe nanocrystals induces strong below band gap absorption as well as a multiphased buildup of bleaching in the 1Se1Sh transition. The amplitudes and kinetics of these features deviate from expectations based on biexciton shifts and state filling, which are the mechanisms usually evoked to explain them. To clarify these discrepancies, the same transitions are investigated here by double-pumpprobe spectroscopy. Re-exciting in the below band gap induced absorption characteristic of hot excitons is shown to produce additional excitons with high probability. In addition, pumpprobe experiments on a sample saturated with single relaxed excitons prove that the resulting 1S(e)1S(h) bleach is not linear with the number of excitons per nanocrystal. This finding holds for two samples differing significantly in size, demonstrating its generality. Analysis of the results suggests that below band edge induced absorption in hot exciton states is due to excited-state absorption and not to shifted absorption of cold carriers and that 1S(e)1S(h) bleach signals are not an accurate counter of sample excitons when their distribution includes multiexciton states.