Plasma resonance absorption

As a further possibility, we can mention that the quantitative intensity of a plasma resonance absorption depends on the third power of the partiele diameter a [151, 72] ... 

The plasma resonance absorption in a cluster as small as Auj might simply be too weak to detect above the background interband absorption, which is quite intense at 520 nm, without the need to invoke any special broadening or damping mechanisms. 

Kawabata, A., Kubo, R. Electronic properties of fine metallic particles. 11 Plasma resonance absorption. J. Phys. Soc. Jpn. 21, 1765-1772 (1966)... 

The optical properties of metal nanoparticles embedded in an insulating host differ substantially from the optical properties of bulk metals. Under the influence of an electrical field, there is a plasmon excitation of the electrons at the particle surface. This resonance, which takes place at a certain energy of the incident light, results in an optical absorption, the so-called plasmon absorption or plasma resonance absorption [1,2]. 

The agreement between the experimental and calculated spectra is rather good. The calculations give a good explanation of the blue shift of the plasma resonance absorption of the silver nanoparticles due to their nanostructural changes during the thermal treatment. In the computations with the... 

Other interpretations of the UV-visible spectroscopy experiments are less plausible as they sometimes contradict other results. Furthermore, it should be mentioned that a plasma resonance absorption has a third power dependence on the particle diameter. Thus, even if the electrons perform a collective motion in the 1 nm particle, the absorption might be too weak to be recognized above the background. 

In this Section we want to present one of the fingerprints of noble-metal cluster formation, that is the development of a well-defined absorption band in the visible or near UV spectrum which is called the surface plasma resonance (SPR) absorption. SPR is typical of s-type metals like noble and alkali metals and it is due to a collective excitation of the delocalized conduction electrons confined within the cluster volume [15]. The theory developed by G. Mie in 1908 [22], for spherical non-interacting nanoparticles of radius R embedded in a non-absorbing medium with dielectric constant s i (i.e. with a refractive index n = Sm ) gives the extinction cross-section a(o),R) in the dipolar approximation as ... 

Figure 6. Absorption spectra of spherical non-interacting nanoclusters embedded in no absorbing matrices (a) effect of the size for Ag nanoclusters in silica (b) effect of the matrix for R = 2.5 nm Au clusters (the refractive index n = and the position of the plasma resonance are reported for each considered matrix) (c) effect of the cluster composition for i = 5 nm noble-metal clusters (Ag, Au, Cu) in silica. (Reprinted from Ref [1], 2005, with permission from Italian Physical Society.)...

Kreibig et al. [68] have recently proposed three alternatives for understanding the lack of a well developed plasma resonance in the UV-visible absorption spectra of AU55 [40, 57,17, 59, 60] which one would normally expect with small metallic particles of gold [69, 70, 71, 72, 73]. These are examined here, together with a fourth possible explanation. 

Marcus et al. [40] argue that this, taken together with a very weak bump around 520 nm in the optical absorption of Au 5 5 in a tetrahydrofuran solution, when compared to similar measurements on colloids, is indicative of collective metallic behavior. 520 nm is the well characterized absorption wavelength for the plasma resonance in colloidal gold for particles of average diameters down to 1 nm [69, 59], showing delocalized electrons with correlated behaviour. 

This behavior differs completely from the discrete one-electron absorptions of low-nuclearity metal cluster molecules [17]. Instead, it resembles the 5d - 6s,6p interband transition of colloidal gold. This demonstrates clearly that the AU55 cluster has electronic energy levels which are closely spaced in a developing band structure, quite similar to colloidal gold. On the other hand, these electrons do not seem to show a collective behavior which would give rise to the plasma resonance. 

The lack of a clearly developed peak in the UV-visible absorption spectrum due to plasma resonance places a limit on the collective metallic behavior of the electrons in the cluster. On the other hand, the 5d -y 6s,6p interband absorption is well developed toward that of large colloidal gold particles. 

Freely mobile conduction electrons normally show a characteristic collective oscillation frequency. This plasma resonance can be observed as an absorption band in the UV-visible spectra of metal colloids. With decreasing particle size... 

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