Optical Properties of Metal Clusters

Da „Goldatome sich sicher optisch anders verhalten als kleine Goldkügelchen“ wäre es „daher wahrscheinlich sehr interessant, die Absorption der Lösungen mit allerkleinsten amikroskopischen Teilchen zu untersuchen and gewissermaßen optisch den Vorgang zu verfolgen, wie sich die Goldteilchen aus den Atomen aufbauen“ [1.1].¹ Mie’s idea of 1908 about the study of optical properties of gold particles with sizes below the resolution limit of light microscopes had to wait more than half a century for its realization, that is until the development in the past two decades of the new field of cluster science.

The electronic structure of metal clusters and particles has been the subject of a large number of investigations (review articles [2.1–17]). It is beyond the scope of this book to discuss all the different theoretical approaches in detail; instead, we will limit ourselves to works which focus on optical properties of metal clusters. A discussion of the history of various theoretical approaches [2.18–20] for understanding the interaction of light with metal spheres can be found, e.g., in [2.21–24].

Until now, we have treated the optical properties of metal clusters theoretically; in this section we introduce experimental techniques. In Chap. 4 results obtained with these techniques will then be presented and discussed with regard to the theoretical approaches in Chap. 2.

Optical properties of clusters have been exploited already for more than one and a half millenia before clusters have been identified as special entities and cluster science been established as an own field of research. As a matter of fact, ancient technologists eagerly produced clusters in glass from empirical recipes (of course not knowning the microscopic structure and processes) because of their beautiful and splendid colors whose brilliance fascinated people. It is not surprising that in particular the purple color of Au and Cu clusters, having been difficult to produce and, hence, rare, were regarded as the emperors color. One of the probably unsurpassed treasures of the antiquity concerning color beauty is a Roman goblet of the fourth century, now to be seen in the British Museum in London, which shows the Greek saga of Lykurgos. Its fame is due to the shining colors generated by a composition of Ag and Au clusters [4.1].