New Transient Absorption Observed in the Spectrum of Colloidal CdSe Nanoparticles Pumped with High-Power Femtosecond Pulses

The power dependence of the transient absorption spectrum of CdSe nanoparticle colloids with size distribution of 4.0 ± 0.4 nm diameter is studied with femtosecond pump-probe techniques. At the lowest pump laser power, the absorption bleaching (negative spectrum) characteristic of the exciton spectrum is observed with maxima at 560 and 480 nm. As the pump laser power increases, two new transient absorptions at 510 and 590 nm with unresolved fast rise (<100 fs) and long decay times (≫150 ps) are observed. The energy of each of the positive absorption is red shifted from that of the bleach bands by ∼ 120 meV. The origin of this shift is discussed in terms of the effect of the internal electric field of the many electron-hole pairs formed within the quantum dot at the high pump intensity, absorption from a metastable excited state or the formation of biexcitons.