state [13] have been postulated. 

One alternative model, which does not require the existence of long-lived charge traps, has been suggested by Frantsuzov and Marcus [14]. In this model, the ‘off’ state arises from the opening of a nonradiative decay channel of the singly excited dot. In the proposed mechanism the excess energy from the trapping of the hole is resonantly transferred to the electron in the lowest excited state (1S) thereby ejecting it to the next excited state (1P). After quickly relaxing back to the 1S state the electron recombines nonradiatively with the trapped hole. Thus, Auger processes are not invoked to account for photodarkening, although these should be observed at an excitation rate exceeding the nonradiative decay rate. The only experimental support to this model obtained so far comes from relatively indirect statistical measurements[15]. 

Most of the experimental work on blinking has focused on characterization of the statistics of ‘on’ and ‘off’ times [3]. The study of the transient decay dynamics of QDs during the ‘on’ and ‘off’ periods is a complementary approach, which has greatly benefited from recent advances in time-resolved photon counting instrumentation. In particular, monitoring the decay dynamics of the remaining fluorescence during ‘off’ periods provides detail on the nonradiative decay mechanism responsible for photodarkening. Such measurements revealed that the emission transient following pulsed excitation is generally nonexponential, and that the emission intensity is correlated with its lifetime [16]. By creating decay curves from only the photons arriving at periods with the highest emission rates, it was shown that the ’on’ state emission is well fit by a single exponent [17]. Later work demonstrated that there exists, in fact, a continuous distribution of emitting states [18] and found a strong correlation between fluorescence intensity and decay times. Very recently, studies on CdSe/CdS QDs with reduced blinking [8, 9] have shown relatively strong emission in the ’off’ state. In both, this was attributed to slower Auger dynamics due to the large QD size. 

Here we attempt to elucidate the microscopic mechanism of QD darkening by a sys- tematic study of the ‘off’ state dynamics in the most studied system of CdSe/CdS/ZnS QDs. In particular, our aim is to clarify the role of the Auger processes, which are known to be strongly dependent on both the nanocrystal size and the excitation intensity. We first study the ‘off’ state fluorescence at low illumination intensities and find that it exhibits size-independent dynamics. This is in clear disagreement with the strong size dependence ex- pected from Auger-assisted photodarkening. We then proceed to characterize the excitation 

