Electron relaxation following UV excitation in CdSe nanocrystals: sub-picosecond-fast population of the 1PStates across a gap wider than 10 phonon energies

The typical energy separation between D-like and P-like electronic states in CdSe nanocrystals is often of similar magnitude to the S-P splitting in the conduction band and would therefore similarly preclude efficient electron decay via multiple phonon emission. Despite the puzzling observation of fast D-to-P intraband relaxation, however, little is known about the mechanisms governing it. This work shows that the same process responsible for fast P-to-S electron decay, namely Auger cooling, plays a fundamental role to allow fast population of the P state, from higher excited states, in case of high-energy excitations. Since in nanocrystals the latter are employed to initiate carrier multiplication, where multiple excitons are created from the absorption of a single photon, the D electron lifetimes calculated here could provide a reference for the estimate of the carrier multipli- cation time constant, the magnitude of which is still object of intense debate. These results also provide a further proof of the suitability of the Auger cooling model to explain electron relaxation in semiconductor nanocrystals.