FORMATION OF UNUSUAL COPPER CLUSTERS IN HELIUM NANODROPLETS

Abstract

Photoabsorption profiles of copper clusters, Cu N, grown in superfluid helium droplets were studied by beam depletion spectroscopy. For N below ~ 103 the spectra are characteristic of individual metallic particles, but in larger clusters a strong, broad absorption feature appears in the near-infrared region. This suggests a transition from single- to multicenter condensation: as the arrival frequency of metal atoms exceeds their transport time within the helium matrix, they nucleate into small individual agglomerates which later collect into a noncompact, possibly fractal or percolative assembly. The data, in concert with earlier observations on silver cluster growth, affirm that the formation of cluster–cluster aggregates with unusual optical properties is a general phenomenon achievable with the use of the helium nanodroplet technique.