Optical response of large lithium clusters: Evolution toward the bulk

Abstract

Multistep photon absorption has been used to measure the collective excitation of free lithium clusters having up to 1500 atoms. The blueshift of the Mie resonance energy, as cluster size increases, probes the surface effects. Its absolute value is consistent with the dielectric constants of the bulk down to a 100 atom cluster. The comparison with calculations in random-phase approximation in the local approximation demonstrates that the jellium model is no longer valid for lithium clusters. PACS numbers: 78.20.Dj, 61.46.+w The optical response of small metal particles as a function of size offers the possibility to follow the development of collective effects in metallic systems. Of these, alkali clusters, and in particular sodium and potassium, are regarded as a prototype of simple metallic particles. Their dipole absorption spectra are consistent with the optical excitation predicted by the classical Mie theory treatment of collective dipole oscillation in spherical or ellipsoidal distorted metallic droplets [1][2][3][4]. A more elaborate approach was obtained from time-dependent density-functional-theory calculations to quantitatively interpret the experimental data on sodium and potassium clusters having less than 40 atoms [5]. However, it was of fundamental interest to explore large cluster sizes to bridge the gap between the free small particles and the bulk in order to know to what extent the macroscopic dielectric function describes the optical response of metallic clusters.We have developed recently an experimental procedure for extending optical absorption measurements [6] to large masses, i.e., a few thousand atoms. For large potassium clusters we have shown that the experimental value of the resonance energy evolves toward the infinite limit h(DM deduced either from the experimental volume plasmon energy hcop [7] or from the surface plasmon energy hcos [8]: hwp^=^ h(OM^^^ hcDs^.Those values are close to the free-electron values. The small difference, less than 5%, which still remains between experimental values and free-electron values is greatly reduced when core polarization and effective mass are included [7,9].Lithium metal differs from the other alkali metals. The measured volume plasmon energy is greatly shifted down from the free-electron value and its linewidth is very broad as compared to other alkali metals [10]. The deviation of the experimental plasmon-peak position from the free-electron value was mainly interpreted in terms of optical effective mass and is due to intraband transitions as pointed out by Paasch [ll]. The volume plasmon linewidth in lithium was interpreted as a plasmon decay via interband transitions [10,12]. So the question arises as to whether such a deviation still exists for clusters. The measurement of the plasmon resonance of lithium clusters is a real challenge to know to what extent the dielectric constants of the bulk are pertinent parameters to interpret the collective excitation of lithium clusters. If so, the microscopic calculation of the dynamic response o...

Gentner‐Kastler‐Preis: Different approaches to cluster spectroscopy

“…It is striking to remark that in a cluster as small as Li 8 , the properties of conducting electrons have already the main characteristics of the bulk. This was confirmed by experiments on larger clusters [9].…”

Oscillatory size dependence of the surface plasmon linewidth in metallic nanoparticles

“…For small clusters the emergence of a non-monotonous size-dependent structure in the lineshape (in experiments 13,14,15 as well as in numerical calculations 10,11,16,17,18 ) has been attributed to shell effects (see also Fig. 10 of Ref.…”

Resonance plasmon linewidth oscillations in spheroidal metallic nanoparticle embedded in a dielectric matrix

“…The damping of surface plasmon resonances in MNs has been well studied both experimentally [12][13][14][15][16][17][18][19][20][21][22][23][24] and theoretically [25][26][27][28][29][30][31][32][33] . Usually, both the bulk and the surface damping mechanisms play an important role in the * email: ngrigor@bitp.kiev.ua surface plasmon decay.…”