Plasmon photoabsorption and surface ionization of metal nanoclusters

UnrestrictedThe ionization and fragmentation of alkali clusters as a result of (1) absorption of ultraviolet light by volume plasmons and (2) collisions with a surface were studied.; The volume plamson is an important elementary excitation, it could not couple to light waves in the bulk and thus could not be investigated by optical spectroscopy, however the situation is different for clusters due to the presence of the boundary. What is more, it is fundamental to study the interaction between light and matter, but the photoabsorption of simple metal clusters in the UV region has remained unexplored experimentally. We have carried out experiments on the photoabsorption of Na₂₀ and Na₉₂ covering the near UV part as well as visible range (~ 2.0 - 5.5 eV) employing the photodepletion technique. The absorption cross sections for clusters of some other sizes in the near UV range have also been studied. The experimental data for Na₂₀ in the visible part agree with existing data very well. In addition to the strong surface plasmon absorption peaks in the visible, we observed appreciable photoabsorption in the UV region. These cross section spectra have been fitted by Lorentzian line shapes and revealed broad peaks slightly above 4 eV. The frequency and weight of these peaks match the theoretical predictions and are identified as the nanocluster "volume plasmon" resonance. The photoabsorption spectra provide the first experimental observation of optically excited "volume plasmon" collective electronic states in metal nanocluster particles, a phenomenon unique to finite systems.; The plasma resonance measurement of pure sodium clusters was extended to mixed clusters in a beam including Na₂₁Cl and Na₂₂Cl₂ and the spectra were compared with that of Na₂₀. For Na₂₀ and Na₂₁Cl, the resonance positions are close, suggesting that the Cl⁻ ion in Na₂₁Cl does not locate at the center of the metallic droplet and does not strongly modify the effective valence electron density. The spectrum of Na₂₂Cl₂ is noticeably dissimilar to the other two, raising the possibility of structural differences and/or incomplete charge transfer.; We also experimentally studied the surface-impact ionization of sodium clusters. Large sodium nanoparticles (~ 10 nm in radius), generated in a vapor condensation source, produce copious amounts of positive ions upon colliding with a surface. The ion flux does not display an exponential surface temperature dependence, is present for both conducting and non-conducting surfaces, and carries a substantial kinetic energy. On the basis of these characteristics it is concluded that the signal arises from dissociative ionization produced by energetic impact, rather than from thermal charge transfer to the surface