  Time-resolved Faraday rotation is used to study both transverse and
longitudinal spin relaxation in chemically-synthesized CdSe nanocrystals (NCs)
22-80 Angstroms in diameter. The precession of optically-injected spins in a
transverse magnetic field occurs at distinct frequencies whose assignment to
electron and exciton spins is developed through systematic studies of the
size-dependence and theoretical calculations. It is shown that the transverse
spin lifetime is limited by inhomogeneous dephasing to a degree that cannot be
accounted for by the NC size distribution alone. Longitudinal spin relaxation
in these NCs occurs on several distinct timescales ranging from 100 ps-10
microseconds and exhibits markedly different dependencies on temperature and
field in comparison to transverse spin relaxation.
