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

in NQDs is dominated by nonphonon energy-loss mechanisms. Recent workshave suggested that coupling to defects [34], Auger interactions with carriersoutside the NQD [35] or Auger-type e–h energy transfer [36] can lead to fastenergy relaxation not limited by a ‘‘phonon bottleneck.’’ The first two mechanismsare not intrinsic to NQDs and cannot explain relaxation data forcolloidal samples [37]. These data indicate that in colloidal nanoparticles,energy relaxation does not show any significant dependence on NQD surfaceproperties (i.e., the number of surface defects) and remains almost identicalfor different liquid- and solid-state matrices, including transparent opticallypassive glasses, polymers, and organic solvents, for which no carriers aregenerated outside the NQD.The energy-loss mechanism proposed in Ref. 36 involves transfer of theexcess energy of an electron to a hole, with subsequently fast hole relaxationthrough its dense spectrum of states. This mechanism is based on the intrinsicAuger-type e–h interactions and leads to significantly faster relaxation timesthan those for the multiphonon emission.Most direct studies of the role of e–h interactions in intraband relaxationhave used CdSe NQDs in which e–h coupling (separation) was controlledby the surface ligand [30,31]. In the presence of hole-accepting capping groups(pyridine in Refs. 30 and 31), the e–h coupling is strong immediately afterphotoexcitation (holes are inside the dot) but is reduced dramatically afterhole transfer to pyridine (hole transfer time s T = 450 fs). Therefore, the rate ofelectron relaxation should be strongly different before and after hole transferto the capping group if this relaxation is indeed due to e–h coupling.In order to monitor electron intraband dynamics at different stages ofhole relaxation/transfer, one can use a three-pulse (pump–postpump–probe),femtosecond, TA experiment [38] schematically shown in Fig. 3. In thisexperiment, the sample is excited by a sequent of two ultrashort pulses [onein the visible and another in the infrared (IR) spectral range] and is probed bybroadband pulses of a femtosecond white-light continuum. The visibleinterband pump is used to create an e–h pair in the NQD, whereas a timedelayed,intraband IR pump is used to resonantly reexcite an electron fromthe 1S to the 1P state. Performing reexcitation at different stages of holetransfer (e.g., before and after the transfer event) and monitoring its relaxationback to the ground state, it is possible to directly evaluate the role ofe–h coupling on electron relaxation rates.Applying the above experiment to a ZnS-overcoated control sample,one does not see any difference in electron dynamics for different reexcitationtimes (Fig. 4a), which is consistent with the fact that a confinement potentialcreated by the ZnS layer inhibits e–h charge separation. In sharp contrast,electron dynamics in the pyridine-capped NQDs (Fig. 4b) show a strongdependence on the delay between visible and IR pulses (Dt IR ). At Dt IR = 70 fsCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.