CHAPTER 1. FLUORESCENT SILICON CLUSTERS 15 Hirsch and co-workers introduced size-selected silicon clusters into an ion trap (Hirsch et al., 2009; Lau et al., 2011; Vogel et al., 2012; Kasigkeit et al., 2015). The trapped clusters were excited by monochromatic synchrotron radi- ation, allowing the determination of core-level shifts for specific cluster sizes. A great variety of geometric structures have been reported for silicon clusters using both theoretical and experimental methods. in general, it has been found that the structures of neutral, cation and anion cluster species differ consider- ably. Anions have frequently been used to elucidate electronic and geometric features. Their structures are affected by Jahn-Teller distortions. Care must be taken when compared to neutral clusters. In a number of early studies, silicon clusters produced using supersonic beam techniques were deposited and investigated spectroscopically. Honea and coworkers deposited size-selected silicon clusters into argon, krypton and nitro- gen matrices using co-deposition onto a liquid helium-cooled substrate (Honea et al., 1993b, 1999). Using surface-plasmon-polariton-enhanced Raman spec- troscopy, sharp features characteristic of Si$_{4}$, Si$_{6}$ and Si$_{7}$ structures were iden- tified in their spectra, which were in good agreement with earlier ab initio cal- culations (Raghavachari and Logovinsky, 1985; Raghavachari, 1986; Tom´ anek and Schl¨ uter, 1986; Raghavachari and Rohlfing, 1988; Ballone et al., 1988). The spectra also revealed evidence for the presence of cluster-cluster aggregation within the rare gas matrix. The spectra of the aggregates of Si$_{4}$, Si$_{6}$ and Si$_{7}$ bore considerable similarities to those of larger clusters, such as Si$_{25}$$_{−}$$_{35}$, as well as those of amorphous silicon (Honea et al., 1999). The structure of free silicon clusters in the size range from n = 10 - 100 was addressed using drift mobility measurements, the results of which revealed a 1.3.2 Geometric structure