CHAPTER 1. FLUORESCENT SILICON CLUSTERS 17 con clusters (Fielicke et al., 2009). Using a combination of tunable far-infrared radiation from the FELIX free electron laser source and vacuum-ultraviolet two-color ionisation, it was possible to scan the spectrum of homonuclear neu- tral silicon clusters in the spectral range from 200 to 550 cm − $^{1}$. The use of vacuum-ultraviolet two-color ionisation provided the advantage of detection of the initially neutral clusters with mass selectivity. An increase of the ionisa- tion rate was observed when IR photons had been absorbed. Comparison with density functional theory (DFT) and Møller-Plesset (MP2) perturbation the- ory calculations revealed that the ground state potential energy surface was very flat. Therefore, rapid interconversion between different structures might well be expected, as well as the presence of higher-energy isomers in real-world experimental samples (Fielicke et al., 2009). Furthermore, theoretical work suggested that, at intermediate sizes, around 20 atoms silicon clusters tend to build irregular cages stabilised by a small num- ber of encapsulated silicon atoms (Mitas et al., 2000). Silicon cages can also be stabilised by encapsulated foreign atoms (Kumar and Kawazoe, 2001, 2003b,a). Stable hollow structures, similar to C$_{60}$ buckminsterfullerene (buckyballs), can nevertheless be excluded (Sun et al., 2003). In summary, the structures of silicon clusters grown in the gas phase differ from the structures of silicon nanocrystals that have been produced, for ex- ample, by etching of bulk crystalline silicon. The structures of small silicon clusters are characterised by the tendency of the atoms to minimise coordina- tion, thereby favouring growth of prolate shapes. Therefore, their bond angles are smaller than their counterparts in sp 3 bonded, cubic diamond-structured bulk silicon. The consequences of this behaviour are shorter internuclear sep- arations and higher atomic densities. An exemption from these principles is shown in the work of Laguna and co-workers, however, who deposited silicon