Metal Cluster Anions Produced by Attachment of Slow Electrons: Readjustment and Blurring of the Magic Numbers

The high electric polarizabilities of metal clusters enable them to attach low-energy electrons with large cross sections: electrons are captured by a strong long-range polarization potential. But the last stage of the collision process is not understood: where and how fast is the captured electron's energy deposited? The question is closely related to the problem of electron relaxation in size-quantized nanosystems. To explore this, we have measured the mass spectra of sodium cluster anions born in the electron-cluster interaction region. If the energy deposited by the captured electron is quickly thermalized and is sufficient to cause rapid cluster evaporation, there should be a rearrangement of the cluster abundances and a shift of the magic numbers from Na$_{n}$ to $\mbox{Na}_{\mbox{n-1}}^{\mbox{-}}$. Such a shift is clearly observed at shell closings near $n$=20 and $n$=40. However, near $n$=58 and $n$=92 the shell closings become completely blurred. This interesting change may be due to a bottleneck in electron relaxation and/or to insufficiently fast evaporation.