Femtosecond 1P-to-1S electron relaxation in strongly confined semiconductor nanocrystals

High-sensitivity femtosecond transient absorption is applied to directly measure the population-depopulation dynamics of the lowest (1S) and the first excited (1P) electron states in CdSe nanocrystals (NC's) of different radii with 1S - 1P energy separation up to 16 longitudinal optical phonon energies. Instead of the drastic reduction of the energy relaxation rate expected due to a phonon bottleneck, we observe a fast subpicosecond 1P-to-1S relaxation, with the rate enhanced in NC's of smaller radius. This indicates the opening of new confinement-enhanced relaxation channels which likely involve Auger-type electron-hole energy transfer.