Femtosecond $1\mathit{P}$-to- $1\mathit{S}$ 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.