Nature of the Second Optical Transition in PbSe Nanocrystals

The second peak in the optical absorption spectrum of PbSe nanocrystals is arguably the most discussed optical transition in semiconductor nanocrystals. Ten years of scientific debate have produced many theoretical and experimental claims for the assignment of this feature as the 1P e1P h as well as the 1S h,e1P e,h transitions. We studied the nature of this absorption feature by pump-probe spectroscopy, exactly controlling the occupation of the states involved, and present conclusive evidence that the optical transition involves neither 1S e nor 1S h states. This suggests that it is the 1P h1P e transition that gives rise to the second peak in the absorption spectrum of PbSe nanocrystals.