The study of simple metal clusters has burgeoned in the last decade, motivated by the growing interest in the evolution of physical properties from the atom to the bulk solid, a progression passing through the domain of atomic clusters. On the experimental side, the rapid development of new techniques for producing the clusters and for probing and detecting them has resulted in a phenomenal increase in our knowledge of these systems. For clusters of the simplest metals, the alkali and noble metals, the electronic structure is dominated by the number of valence electrons, and the ionic cores are of secondary importance. These electrons are delocalized, and the electronic system exhibits a shell structure that is closely related to the well-known nuclear shell structure. In this article the results from a broad range of experiments are reviewed and compared with theory. Included are the behavior of the mass-abundance spectra, polarizabilities, ionization potentials, photoelectron spectra, optical spectra, and fragmentation phenomena.

The physics of simple metal clusters: experimental aspects and simple models

The jellium model of simple metal clusters has enjoyed remarkable empirical success, leading to many theoretical questions. In this review, we first survey the hierarchy of theoretical approximations leading to the model. We then describe the jellium model in detail, including various extensions. One important and useful approximation is the local-density approximation to exchange and correlation effects, which greatly simplifies self-consistent calculations of the electronic structure. Another valuable tool is the semiclassical approximation to the single-particle density matrix, which gives a theoretical framework to connect the properties of large clusters with the bulk and macroscopic surface properties. The physical properties discussed in this review are the ground-state binding energies, the ionization potentials, and the dipole polarizabilities. We also treat the collective electronic excitations from the point of view of the cluster response, including some useful sum rules.

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The physics of simple metal clusters: self-consistent jellium model and semiclassical approaches

3.1. Enhanced Electronic Excitations 3968 3.2. Surface-Enhanced Infrared Absorption 3970 4. Surface-Enhanced Fluorescence 3970 5. Surface-Enhanced Raman Scattering 3974 5.1. Early History of SERS 3974 5.2. The Electromagnetic Mechanism 3975 5.3. The Chemical Mechanism 3976 5.3.1. The Resonance Raman Mechanism 3976 5.3.2. The Charge-Transfer Mechanism 3977 5.3.3. The Nonresonant Chemical Mechanism 3977 5.4. Unifying the Description of SERS 3978 5.4.1. Combined Quantum and Classical Method for SERS 3979 6. Chiroptical Properties of Small Metal Clusters 3979 6.1. Circular Dichroism 3979 6.1.1. Chiral Core Model 3980 6.1.2. Dissymmetric Field Model 3981 6.1.3. Chiral Footprint Model 3981 6.2. Vibrational Raman Optical Activity 3983 7. Nonlinear Optical Properties 3984 7.1. Surface-Enhanced Hyper-Raman Scattering 3984 8. Conclusion 3986 Author Information 3987 Biographies 3987 Acknowledgment 3987 References 3988

Theoretical studies of plasmonics using electronic structure methods

The optical-absorption spectra of small mass-selected ${\mathrm{Ag}}_{\mathit{N}}$ clusters (N=2\char21{}21) embedded in solid argon are measured in the energy range 2.5\char21{}6.2 eV. Investigation of a continuous range of cluster sizes reveals the size development of the photoabsorption behavior. The main features of the spectra are in agreement with predictions based on the Nilsson-Clemenger shell model. The measured photoabsorption cross sections are consistent with a sum-rule calculation involving only s electrons. In addition, the medium influence on the spectra is studied by changing the matrix gas from argon to krypton and xenon for the cluster sizes N=7, 11, 15, and 21.

Collective dipole oscillations in small silver clusters embedded in rare-gas matrices.

Photodepletion spectroscopy is used to measure the optical activity of sputtered Ag ionic clusters with up to 70 atoms. With decreasing cluster size the giant resonance caused by collective excitation of the valence electrons shifts to higher frequencies. This blue shift is qualitatively explained in terms of a reduced s-d screening interaction in the surface region of the particles.

Blue shift of the Mie plasma frequency in Ag clusters and particles

Absorption spectra of lithium clusters containing four to eight atoms have been measured using depletion spectroscopy. Few intense transitions are observed, always located in two predominant spectral regions, ∼480 and 680 nm. The spectra are interpreted using ab initio configuration interaction (CI) calculations, leading to a complete characterization of the excited states and a straightforward determination of the ground state geometrical structure. Intense transitions are explained by interference effects in the transition amplitude and symmetry considerations. Comparisons with semiclassical models, in which an effective mass correction is introduced, are also presented.

Evolution of the electronic structure of lithium clusters between four and eight atoms

Evolution of the Electronic Structure of Lithium Clusters Between Four and Eight Atoms

The optical-absorption spectrum of Lib has been observed by depletion spectroscopy in the 400-700-nm range. The experimental results are compared with theoretical spectra calculated for the three Li 6 structures in competition and we have been able to unambiguously determine the most stable Li 6 geometry as being the C 2υ tripyramidal geometry

Competition between planar and nonplanar structure in alkali hexamers: The example of Li6.

The optical absorption spectrum of small lithium clusters has been measured up to Li8. In Li3 high resolution Two Photon Ionization (TPI) spectra have been recorded allowing us to determine the geometry and potential surfaces of the ground and excited states. In larger clusters, the excited states are dissociative and the absorption spectra have been obtained by Depletion Spectroscopy. Vibronic resolution is still achieved in Li4, but not in larger clusters. The measured spectra exhibit a rather small number of transitions to electronically excited states. In Li7, only one intense band is observed in the blue region, while in Li8, an intense band is also observed in the blue region and a much weaker band in the red region. All the obtained results are in very good agreement with the ab initio calculation of Bonacic-Koutecky et al. This demonstrates that molecular effects are always present in these small clusters. The semi-classical models of surface plasma resonances are also discussed.

High resolution spectroscopy of small metal clusters

We have measured the single and double photoionization efficiency of Hgn mass selected clusters as a function of photon energy between 30 and 110 eV for masses ranging from n=1 to 36 for the singly ionized and from n=5 to 70 for the doubly ionized clusters, using the light beam issued from a wiggler at the BESSY synchrotron. The results show in all cases a spectrum similar to the 5d→ef shape resonance previously observed in the isolated atom. In contrast, the expected 5p→6p transition allowed by hybridization of the 6s and 6p orbitals is not observed. We also compared the mass spectra of singly and doubly charged mercury clusters ionized by electron impact (30–90 eV) to those of clusters ionized by synchrotron radiation (30–70 eV). We found that the ratio Hg2+n/Hg+n was higher for photoionization at low energy and for electron impact ionization at high energy. Finally, we estimated the critical stability size of the triply charged clusters.

Photoionization spectroscopy of small mercury clusters in the energy range from vacuum ultraviolet to soft x ray

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