Photoabsorption of ${\mathrm{Ag}}_N(N\sim 6–6000)$ Nanoclusters Formed in Helium Droplets: Transition from Compact to Multicenter Aggregation

${\mathrm{Ag}}_N$ clusters with up to thousands of atoms were grown in large He droplets and studied by optical spectroscopy. For $N\lesssim {10}^3$ the spectra are dominated by a surface plasmon resonance near 3.8 eV and a broad feature in the UV, consistent with absorption by individual metallic particles. Larger clusters reveal unexpectedly strong broad absorption at low frequencies, extending down to $\approx 0.5\mathrm{eV}$. This suggests a transition from single-center to multicenter formation, in agreement with estimates of cluster growth kinetics in He droplets. Moreover, the spectra of large clusters develop a characteristic dispersion profile at 3.5–4.5 eV, indicative of the coexistence of localized and delocalized electronic excitations in composite clusters, as predicted theoretically.

Figure 1

Normalized photoabsorption spectra for Ag particles of different average sizes assembled by pickup in He droplets. Dashed lines in panels (b) and (c) are calculated model cross sections (right-hand scale) and in panel (e) is the fit of the dispersion profile according to the Fano formalism. The top panel (f) shows the calculated absorption spectrum of an Ag cluster-cluster aggregate from Ref. 23 (see text for details).

Figure 2

Time between two successive pickup events, $t_{n,n+1}$, and time required for recombination of first two picked up Ag atoms, $t_{\mathrm{rec}}$, versus size of He droplets, $N_{\mathrm{He}}$. Corresponding droplet radius, $R_{\mathrm{He}}$, and typical size of ${\mathrm{Ag}}_N$ cluster, $N_{\mathrm{Ag}}$, grown within the droplet are also shown on bottom and top axes of plot, respectively.