  The rates of elementary photophysical processes in nanocrystals, such as
carrier cooling, multiexciton generation, Auger recombination etc., are
determined by monitoring the transient occupation of the lowest exciton band.
The underlying assumption is that hot carriers relax rapidly to their lowest
quantum level. Using femtosecond transient absorption spectroscopy in CdSe/CdS
nanodots we challenge this assumption. Results show, that in nanodots
containing a preexisting cold exciton "spectator", \emph{only half of the
photoexcited electrons}relax directly to the band-edge and the complementary
half is blocked in an excited state level due to Pauli exclusion. Full
relaxation occurs only after \textasciitilde 15 ps, as the blocked electrons
flip spin. This novel spin-blockade effect may offer the key for the
long-sought-for bottleneck mechanism for multiexciton energy dissipation.
