  Silicon nanocrystals are studied by time-resolved fluorescence spectroscopy.
After laser excitation the bright and dark exciton ground state levels are
populated at random, but subsequently the decay curves reveal a thermalization
between these levels. The characteristic thermalization time is found to be
approximately 100 ns for temperatures below 100 K and surprisingly increases
for higher temperatures. The decay curves are analyzed using a simple two-state
model for the bright and dark exciton ground states.
