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

Figure 3 Schematic of a three-pulse (pump–postpump–probe) experiment and theexcitation/relaxation processes monitored in it. An e–h pair is created via an interbandexcitation with a pump pulse in the visible spectral range. The electron is reexcitedwithin the conduction band by an IR postpump pulse. The electron relaxationback to the ground state is monitored with a third pulse, probing interband absorptionchanges.(diamonds in Fig. 4b), the relaxation constant is 250 fs, which is close to thevalue measured for ZnS-capped NQDs. With increasing reexcitation time, theelectron relaxation time gradually increases up to 3 ps at Dt IR = 430 fs (trianglesin Fig. 4b). A further increase in Dt IR does not lead to significantchanges in electron intraband dynamics [compare traces taken at Dt IR = 430fs (triangles) and 600 fs (squares); Fig. 4b]. Importantly, the threshold delay ofroughly 400 fs found in these experiments is very close to the hole-transfertime to a capping molecule inferred from two-pulse TA measurements [31].This observation indicates that dramatic changes in electron relaxation resultfrom hole transfer to the surface pyridine group, strongly suggesting thatelectron relaxation is dominated by e–h interactions (Auger-type e–h energytransfer).Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.