Electron-plasmon interaction

In atomic-molecular media the damping of plasmon states is due to the interaction of plasmon waves with electrons, lattice vibrations, and impurities. The electron-plasmon interaction is a long-range one. With absorption of a plasmon, the momentum q is transferred to the electron, resulting in a decay of the collective state into a single-particle one. The latter process is identical with absorption of a photon with the same energy. Wolff102 (see also Ref. 103) has shown that in this case the lifetime can be expressed in terms of two optical constants the absorption coefficient k and the refractive index nT, namely,... 

Free carriers change Raman spectra, either by single particle contribution to the spectrum, or by phonon- plasmon interaction. In addition, interference of electronic transition continua with single phonon excitations may lead to Fano line shapes, as mentioned in the introduction. The Fano effect is encountered in p-doped Si crystals, as shown in Fig. 4.8-19. The shown lines correspond to the respective Raman active mode at 520 cm for crystals with 4 different carrier concentrations, excited with a red laser. The continuous line is calculated according to Eq. 4.8-6. Antiresonance on the low frequency side and line enhancement on the high frequency side are a consequence of the positive value of Q. A reverse type of behavior is possible in the case of a negative Q. 

Electrons also interact inelastically with the sample. Incident electrons can excite plasmons and phonons, and atom-bonded electrons can be excited from the ground state to higher unoccupied electron states. In the de-excitation process, i.e., when the excited electron returns to the unoccupied core state (Fig. 2),... 

Fig, 74. (a) LA phonon dispersion in the [111] direction for intermediate-valent TmSe. An electronic plasmon mode is shown as the dashed horizontal line. Mixed electronic plasmon-phonon modes are shown with dotted lines. The heavy dashed line is the phonon mode without interaction, (b) Phonon density of states with (solid curve) and without (dashed curve) plasmon interaction. (After Treindl and Wachter 1980.)... 

Such limitations do not apply in the field of metamaterials since here plasmons interact with electromagnetic fields. A plasmon is a free-electron oscillation in a metal and exhibits quite distinct photonic behavior from a photon. Generally, for the same frequency, the wavelength of a plasmon is much smaller than that of a photon. For this reason, metamaterials with small lattice dimensions can still... 

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... 

There are several other techniques Uke the fluorescent dye displacement assays, footprinting, Fourier transform infrared spectroscopy. X-ray crystallography, electron microscopy, confocal microscopy, atomic force microscopy, surface plasmon resonance etc used for hgand-DNA interactions that are not discussed here. 

As already briefly mentioned in the introduction, some metals exhibit so-called plasmon resonances in the UV-visible spectra, attributed to the interaction of electromagnetic waves (visible light) and the confined electron gas, if a critical size on the nanoscale is reached. The process is sketched in a simplified manner in Figure 8. 

The appearance of a plasmon resonance is strictly related to a distinct size of the corresponding metal, based on the presence of a confined electron gas that interacts with light and so results in typical colours. Is there also a minimum size where plasmon resonance is no longer possible In any case this must happen if a particle reaches a typical molecular status. There are no longer freely mobile... 

Figure 8. Illustration of the interaction visible light and the confined electron gas of a metal nanoparticle, resulting in a plasmon resonance.

The reaction was studied for all coinage metal nanoparticles. In the case of GMEs the rate follows zero-order kinetics with IT for all the coinage metal cases. The observed IT for the Cu catalyzed reaction was maximum but its rate of reduction was found to be minimum. Just the reverse was the case for Au and an intermediate value was obtained for the Ag catalyzed reaction (Figure 7). The adsorption of substrates is driven by chemical interaction between the particle surface and the substrates. Here phe-nolate ions get adsorbed onto the particle surface when present in the aqueous medium. This caused a blue shift of the plasmon band. A strong nucleophile such as NaBH4, because of its diffusive nature and high electron injection capability, transfers electrons to the substrate via metal particles. This helps to overcome the kinetic barrier of the reaction. 

When the size of metals is comparable or smaller than the electron mean free path, for example in metal nanoparticles, then the motion of electrons becomes limited by the size of the nanoparticle and interactions are expected to be mostly with the surface. This gives rise to surface plasmon resonance effects, in which the optical properties are determined by the collective oscillation of conduction electrons resulting from the interaction with light. Plasmonic metal nanoparticles and nanostructures are known to absorb light strongly, but they typically are not or only weakly luminescent [22-24]. 

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