Publication date: 27th June 2014
A novel ultrafast transient absorption spectroscopy is employed to detect and quantify multi exciton generation (MEG) in lead selenide (PbSe) nanocrystals. This involves comparing signals obtained in “equivalent” experiments conducted with two different pump photon energies one below and the other well above the theoretical threshold for MEG, at 800 and 400 nm respectively. This “equivalence” involves production a similar distribution of absorbed photons at both photon energies. This can be guaranteed by preparing samples of identical optical density at each pump wavelength and scaling the pump pulse intensities inversely the extinction coefficient. Thus ambiguity due to direct multi-photon excitation, uncertainties of absolute absorption cross sections, and low signal levels are overcome. The results show that a single scaling factor, related to the ratio of the samples absorption cross sections at both excitation wavelengths, brings the spectral and temporal cuts after 400nm and 800nm into perfect overlap. In particular, no excess in bleach is observed at early times after 400 nm excitation, which would be indicative of the presence of extra carrier. The satisfactory match of spectra at all delays through the described scaling demonstrates the absence of carrier multiplication up to a photon energy level of 3.7 times the band gap. Beyond the issue of carrier multiplication, Following the Exciton cooling during the first few picoseconds reveals; 1) at both pump wavelengths, the entire transient spectrum seems at early delays to be dominated by a broad absorptive background extending throughout the probed range. 2) The apparent background is broader when exciting with the more energetic photons. 3) The amplitude of the localized bleaching component at the band edge is nearly conserved during the process of cooling independent of the exciting photon energy.
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