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 nanoparti cle colloids with size distribution of 4.0 +/- 0.4 nm diameter is studied w ith femtosecond pump-probe techniques. At the lowest pump laser power, the absorption bleaching (negative spectrum) characteristic of the exciton spec trum is observed with maxima at 560 and 480 nm, As the pump laser power inc reases, two new transient absorptions at 510 and 590 nm with unresolved fas t rise (<100 fs) and long decay times (much greater than 150 ps) are observ ed. The energy of each of the positive absorption is red shifted from that of the bleach bands by similar to 120 meV. The origin of this shift is disc ussed in terms of the effect of the internal electric field of the many ele ctron-hole pairs formed within the quantum dot at the high pump intensity, absorption from a metastable excited state or the formation of biexcitons.