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

Figure 19 Magnetic field dependence of (a) emission decays recorded at the peakof the luminescence and (b) FLN spectra excited at the band edge (2.467 eV) for 1.2-nm-radius CdSe nanocrystals. The FLN spectra are normalized to their one phononline. A small amount of the excitation laser is included to mark the pump position.Experiments were carried out in the Faraday geometry (magnetic field parallel to thelight propagation vector). (Adapted from Ref. 10.)the excited nanocrystal must return to the ground state from the dark exciton.The long (As) emission is consistent with recombination from this weaklyemitting state. However, because a strong magnetic field couples the darkexciton to the optically allowed sublevels, the emission lifetime should decreasein the presence of a magnetic field. Because the experimental fluorescencequantum yield remains essentially constant with field, this mixing leadsto the decrease in the emission decay with increasing magnetic field [10].Another peculiar effect which can easily be explained by the darkexciton is the influence of a magnetic field on the vibrational spectrum, whichis demonstrated in Fig. 19b. A dramatic increase is observed in the relativestrength of the zero-phonon line with increasing field. This behavior resultsfrom the dark exciton utilizing the phonons to relax to the ground state. In asimplistic picture, the dark exciton would have an infinite fluorescence lifetimein zero applied field because the photon cannot carry an angular momentumof 2. However, nature will always find some relaxation pathway, no matterhow inefficient. In particular, the dark exciton can recombine via a LOphonon-assisted,momentum-conserving transition [81]. In this case, thehigher-phonon replicas are enhanced relative to the zero-phonon line. If anexternal magnetic field is applied, the dark exciton becomes partially alloweddue to mixing with the optically allowed sublevels. Consequently, relaxationno longer relies on a phonon-assisted process and the strength of the zerophononline increases.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.