Collective plasmon

Although as far back as 1960 Fano92 has pointed out the possibility of existence of collective plasmon-type oscillations in molecular media, this question has been discussed for quite a long time (see Ref. 25), especially as Platzman17 has shown that Fano s criterion of existence of plasmons, namely,... 

Thus, the excitation of collective plasmon-type states is one of the important mechanisms of energy loss by fast particles in thin layers of condensed matter, the thickness of which is much smaller than the path length of the particles. However, this is not necessarily so with thick absorbents, as it is sometimes erroneously stated (see, for instance, Ref. 3). 

The latter effect originates from the coupling between the incident and scattered radiation with localized and/or collective plasmon resonances in the rough metal film. Therefore the intensity of the totally symmetric Ag modes is very sensitive to the morphology of the metal film. 

Conditions for collective, plasmon-type excitations will be addressed in Section 2.6. It will be shown that they can be fulfilled only for very large systems. Future prospects will be given in the concluding remarks. 

The size dependence of the total widths has been studied by several theoretical groups [8, 37, 40, 59-61]. The jellium model was used nearly exclusively. In this case, the interband decay of the plasmon discussed above is not possible, and the collective plasmon oscillations can only decay by exciting a single electron from the same band — a process that has been termed Landau damping . For sufficiently large clusters this gives (Equation 9.7 of Ref. [37]) a width like... 

Chen F, Alemu N, Johnston RL. Collective plasmon modes in a compositionally asymmetric nanoparticle dimer. AIPAdv 2011 1 032134. 

Most clusters and nanoparticles studied with time-dependent methods are still of quite simple nature, e.g. metal clusters or semiconductor nanoparticles. These systems consist of only a few elements. Several metal clusters exhibit collective plasmon excitations, and semiconductor nanoparticles are known to be able to generate long-lived excitons. To show and explain these is the task of time-dependent methods that are employed to calculate their photo absorption spectra. 

Electron energy loss experiments give information about the gap states, the position and shape of the deep level absorption thresholds and the collective plasmon excitations. The gap widths of simple oxides typically range from 3 to 10 eV ... 

The dynamic response of the electrons to a time-varying (m f 0) homogeneous q — 0) perturbation is determined by e q 0,(u). The elementary electronic excitations of the systems are of two types the inter-band transitions, associated with the creation of electron-hole pairs, which correspond to the poles of e q — 0,< ) and the collective plasmon excitations. The inter-band transitions with the lowest energies are produced when an... 

Figure Bl.25.6. Energy spectrum of electrons coming off a surface irradiated with a primary electron beam. Electrons have lost energy to vibrations and electronic transitions (loss electrons), to collective excitations of the electron sea (plasmons) and to all kinds of inelastic process (secondary electrons). The element-specific Auger electrons appear as small peaks on an intense background and are more visible in a derivative spectrum.

Valence electrons also can be excited by interacting with the electron beam to produce a collective, longitudinal charge density oscillation called a plasmon. Plas-mons can exist only in solids and liquids, and not in gases because they require electronic states with a strong overlap between atoms. Even insulators can exhibit... 

Figure 2.36 A shows a typical low-loss spectrum taken from boron nitride (BN). The structure of BN is similar to that of graphite, i. e. sp -hybridized carbon. For this reason the low-loss features are quite similar and comprise a distinct plasmon peak at approximately 27 eV attributed to collective excitations of both n and a electrons, whereas the small peak at 7 eV comes from n electrons only. Besides the original spectrum the zero-loss peak and the low-loss part derived by deconvolution are also drawn. By calculating the ratio of the signal intensities hot and Iq a relative specimen thickness t/2 pi of approximately unity was found. Owing to this specimen thickness there is slight indication of a second plasmon.

In dielectric materials (oxides, semiconductors, halides, polymers, and he like), polarizability correlates with hardness. For metals, this is not the case. However, the frequencies of the collective polarizations known as plasmons are related to mechanical hardness. 

One source of EM enhancement may be attributed to the excitation of surface plasmons (SP) in the metal. A plasmon is a collective excitation in which all of the conduction electrons in a metal oscillate in phase. In the bulk, there is essentially only one allowed fundamental plasmon frequency. 

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