and Constant, 1991; Bower and Jarrold, 1992; Honea et al., 1993a, 1999). Sput- ter gas aggregation sources employ a modified sputter electrode configuration, allowing higher operating gas pressures than usually needed in sputtering for thin film production (Haberland et al., 1991; Astruc Hoffmann et al., 2001a; Pratontep et al., 2005; Wegner et al., 2006; von Haeften et al., 2009). Argon and helium are used as seed gases. Helium produces smaller clusters because it is much lighter than silicon and energy transfer is less efficient. The pulsed arc cluster ion source is another variation of a gas aggregation-type cluster source that has been used for silicon cluster production (Maus et al., 2000). 

These sources produce neutral and ionic clusters, including both cations and anions; charged clusters can be easily manipulated in a spatial sense via elec- trostatics, and can therefore be mass-selected. Kitsopoulos and co-workers size- selected silicon cluster anions, which were then photoexcited into the neutral ground state as well as other low-lying electronically excited states of the neutral clusters by the process of electron detachment (Kitsopoulos et al., 1990). Honea and co-workers used arrangements of linear and perpendicular quadrupole mass selectors to size-select silicon cluster ions and deposit them onto a surface (Honea et al., 1999). To soften the impact, rare gas buffer layers were co-deposited onto a cold, pre-deposited rare gas matrix (Honea et al., 1999). A similar scheme was used by Grass and co-workers, who soft-landed Si$_{4}$ on highly oriented pyrolytic graphite (HOPG) and performed X-ray photoelectron spectroscopy (Grass et al., 2002). Astruc-Hoffmann and co-workers used a reflectron time-of- flight (RETOF) mass spectrometer in combination with a multi-wire mass gate to size select silicon anion clusters on which to subsequently perform photo- electron spectroscopy. The size-selected clusters were irradiated with UV light from an ArF excimer laser and analysed in a magnetic bottle photoelectron spectrometer (Astruc Hoffmann et al., 2001a). 

