  To understand and optimize optical spin initialization in room temperature
CdSe nanocrystal quantum dots (NCQDs) we studied the dependence of the
time-resolved Faraday rotation signal on pump energy $E_p$ in a series of NCQD
samples with different sizes. In larger NCQDs, we observe two peaks in the spin
signal vs. $E_p$, whereas in smaller NQCDs, only a single peak is observed
before the signal falls to a low, broad plateau at higher energies. We
calculate the spin-dependent oscillator strengths of optical transitions using
a simple effective mass model to understand these results. The observed $E_p$
dependence of the spin pumping efficiency (SPE) arises from the competition
between the heavy hole (hh), light hole (lh) and split-off (so) band
contributions to transitions to the conduction band. The two latter
contributions lead to an electron spin polarization in the opposite direction
from the former. At lower $E_p$ the transitions are dominated by the hh band,
giving rise to the low energy peaks. At higher $E_p$, the increasing
contributions from the lh and so bands lead to a reduction in SPE. The
different number of peaks in larger and smaller NCQDs is attributed to
size-dependence of the ordering of the valence band states.
