  Using time-resolved photoluminescence spectroscopy, we studied the electronic
levels of semiconductor nanocrystals (NCs) with small spin-orbit coupling such
as CdS. Low temperature radiative rates indicate that the lowest energy
transition changes from orbital allowed to orbital forbidden with decreasing NC
size. Our results are well explained by a size-dependent hierarchy of s- and
p-orbital hole levels that is in agreement with theoretical predictions. Around
the critical NC radius of ~2 nm, we observe an anti-crossing of s- and
p-orbital hole levels and large changes in transition rates.
