Photoabsorption by Volume Plasmons in Metal Clusters*

Metal clusters exhibit strong photoabsorption resonances in the visible part of the spectrum. These are collective ``surface plasmon'' excitations which have been extensively investigated. However, the UV part of the spectrum has remained rather unexplored. Some theoretical calculations have predicted that a measurable portion of the delocalized valence electrons' dipole oscillator strength should be located in this region. This predicted absorption feature has been ascribed to a volume-plasmon type of excitation, which in small particles can couple to light, in contradistinction to the situation in bulk metals. We have carried out a photodepletion cross section measurement on a pair of prototypical simple-metal nanoclusters, Na$_{20}$ and Na$_{92}$, finding that these systems indeed possess a broad volume-plasmon absorption peak centered at $\approx $4 eV and having an oscillator strength contents of $\approx $15-20{\%} of the total, in good agreement with theoretical calculations. These spectra provide the first experimental confirmation of the existence of optically active volume-type collective electronic excitations in metal nanocluster particles.