  Electron-phonon relaxation in size-quantized systems may become inhibited
when the spacing of discrete electron energy levels exceeds the magnitude of
the phonon frequency. We show, however, that nanoclusters can support a fast
nonradiative relaxation channel which derives from their distinctive ability to
undergo Jahn-Teller shape deformations. Such a deformation represents a
collective and coherent vibrational excitation and enables electronic
transitions to occur without a multiphonon bottleneck. We analyze this
mechanism for a metal cluster within the analytical framework of a
three-dimensional potential well undergoing a spheroidal distortion. An
expression for the time evolution of the distortion parameter is derived, the
electronic level crossing condition formulated, and the probability of
electronic transition at a level crossing is evaluated. An application to
electron-hole recombination in a closed-shell aluminum cluster with 40
electrons shows that the short (~250 fs) excitation lifetime observed in recent
pump-probe experiments can be explained by the proposed mechanism.
