Decades of investigation show that inter-band photoexcitation of quantum dots is followed by rapid relaxation of hot carriers to the quantized band edge states within one or two picoseconds. Due to the large oscillator strength and low degeneracy of the band edge exciton transition, evolution in its intensity and spectrum have played a pivotal part in probing quantum dot exciton cooling. These changes start as a bi-exciton spectral shift while carriers are hot, changing to a bleach due to state filling once the exciton relaxes. Accordingly, kinetics of the BE bleach buildup has served to characterize the final stages of carrier cooling, and its amplitude per cold exciton the degeneracy of underlying electronic states.

Hot multi-excitons (MX) add a new relaxation process to this scenario. Auger recombination (AR) reduces an N exciton state to N-1 plus heat, initially. Again, investigation of AR dynamics is based on the amplitude and decay kinetics of the BE bleach. Interpretation of such data was based on the following assumptions: 1) that ultrafast cooling of hot excitons leads directly to occupation of the lowest electron and hole states (in accordance with the lattice temperature and the state degeneracy), and 2) that aside from mild spectral shifts induced in the remaining band edge transitions, after carrier cooling is over the BE bleach increases linearly with N until state filling is complete.

To test these assumptions, three pulse pump-probe experiments were conducted in our lab, measuring fs transient transmission (TT) of PbSe nanocrystals in the presence and absence of single cold spectator excitons. Results show that the bleach introduced by a second hot exciton falls significantly from that introduced by the spectator. Later we will describe a recent extension to CdSe which shows that adding an additional hot exciton to the cold spectator reduces the band edge bleach merely by a half! We conclude that the source of incomplete bleaching by the second exciton are hitherto unrecognized random spin orientation conflicts between the two conduction electrons in the crystallites. The presence of this effect both in lead salts and in CdSe NCs demonstrates its generality. This new discovery imposes new restrictions on the utility of the BE exciton transition as a universal “exciton counter” in experiments with all kinds of semiconductor NCs