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We report collisional broadening of the E3 excitonic resonances in opticalabsorption spectra of PbS nanocrystallites of widely varying sizes.Significance of the underlying extended band structure of bulk solids tounderstand the physics of exciton scattering in semiconductor nanocrystallitesof this size range is discussed. We propose an empirical notion of 'effectiveBohr exciton radius' as a direct consequence of significant departure fromusual static screening limits of coulomb interactions of 'hot' excitons in theregion of strong dispersion at energies much above the fundamental band gap.Temperature variation of excitonic resonance reveals how non-phonon energyrelaxation processes affects the collisional broadening of E3 resonance overthis size range. We argue that ballistic transport of hot excitons can suppressexciton-exciton scattering necessary for photo-induced impact ionization invery small quantum dots. This evidently indicates that impact ionization of hotexcitons may be efficient only inside an intermediate nanoscale 'size window'.

Do we need to revisit the Bohr exciton radius of hot excitons?