Origin of one-photon and two-photon absorption peaks in PbSe nanocrystals from atomistic pseudopotential calculations

PbSe nanocrystals represent the paradigm nanoscale system exhibiting carrier multiplication upon light absorption, yet the origin of the absorption peaks remains elusive. The second peak in the one-photon absorption spectrum has been often attributed to dipole-forbidden P-S and S-P inter-band transitions. Recent two-photon photo-luminescence excitation experiments have shown that the S-P and P-S transitions, which are selectively excited by two-photon absorption, are indeed close in energy to the second one-photon absorption peak. Here we report atomistic pseudopotential calculations of the one-photon and two-photon absorption spectra of PbSe nanocrystals 30.6 {\AA} in radius. We find that the main two-photon absorption peak indeed originates from P-S and S-P transitions, and is blue-shifted by 36 meV with respect to the second one-photon absorption peak, in excellent agreement with experiment. However, our calculations show unequivocally that, contrary to previous interpretations, the second one-photon absorption peak originates from dipole-allowed P-P transitions.