Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

where x = cos h and f = l B g e H/3g. The probability of exciton recombinationincreases in weak magnetic fields (f1), reaching3ð1 cos h H Þ 3g14s 0 l B g e H ð1 þ cos h HÞ :One can see from Eqs. (46) and (51) that the recombination lifetimedepends on the angle between the crystal hexagonal axis and the magneticfield. The recombination time is different for different crystal orientations,which leads to a nonexponetial time decay dependence for a system ofrandomly oriented crystals.IV.EXPERIMENTThe fine structure of band-edge exciton spectra explains various unusual andunexpected properties of CdSe NC ensembles, including the linear polarizationmemory effect [20], circular polarized PL of a single CdSe nanocrystal[32], linear polarized PL of individual CdSe nanorods [33], Stokes shift of theresonant PL [2,3], fine structure of the resonant PL excitation spectra of CdSeNCs [34], shortening of the radiative decay time in magnetic field [2,3],magnetocircular dichroism [35], and polarization of the PL in a strong magneticfield [36]. Here, we will briefly discussed these results and provide theirqualitative and quantitative explanations.A. Polarization Properties of the Ground Dark-Exciton StateAs we have already discussed, the ground exciton state in CdSe NCs does nothave a dipole moment and it cannot emit light in electric-dipole approximation.The radiative recombination of the dark exciton can only occur throughsome assisting processes that flip the electron-spin projection or change thehole angular momentum projection [12]. As a result, the polarizationproperties of the low-temperature PL are determined by polarization propertiesof virtual optical transitions that are activated by phonons. The opticalspherical phonons with the angular momenta 0 and 2 can participate in thesetransitions [22,23]. The phonons with the angular momentum 2, for example,Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.