We have measured absolute cross sections for the capture of low-energy electrons by large free sodium nanoclusters ( $\sim {10}^4\mathrm{atoms}$, or 4 nm radius). To explain the results, it is necessary to go beyond the commonly used induced-dipole approximation and to employ the full image-charge interaction potential which accounts for the finite size of the particle. This potential yields an exact analytical expression for the capture cross section and leads to good agreement with the data. It is suggested that electron capture may provide a useful tool for size characterization of nanoparticle beams.

• Received 6 June 2000

DOI:https://doi.org/10.1103/PhysRevLett.85.2729