Electron relaxation following UV excitation in CdSe nanocrystals: sub-picosecond-fast population of the 1P states 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 the 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 multiplication time constant, the magnitude of which is still the object of intense debate. These results also provide further proof of the suitability of the Auger cooling model to explain electron relaxation in semiconductor nanocrystals.