Novel spectral decay dynamics of hot excitons in PbSe nanocrystals: A tunable femtosecond pump-hyperspectral probe study

Ultrafast exciton cooling in highly monodisperse PbSe nanocrystals is followed with tunable pump-hyperspectral near-IR probe spectroscopy. Unexpected kinetic and spectral correlations between induced bleach and absorption features are revealed, which are incompatible with standard models for excited nanocrystal absorption. Interband optical excitation immediately generates a sharp bleach feature near the 1S_h1S_e transition which is unchanged during exciton thermalization, while pumping well above the band edge induces an intense absorption at frequencies just below the band edge which decays concurrently with a buildup of renewed absorbance at the 1P _h1P_e peak during exciton cooling. Transient spectra of hot single and double excitons are nearly indistinguishable, arguing against the controversial involvement of Auger cooling in the rapid dissipation of excess energy in excited PbSe QDs. Finally, quantitative signal analysis shows no signs of multiexciton generation up to photon energies four times the sample band gap.