content_image/295693.jpg
<img src='content_image/295693.jpg'>

shown in figure 1.4 illustrates this mechanism. 

In this example, the molecule performs a bend vibration. At the classi- cal turning points of this motion, structures belonging to three different point groups, D$_{inf}$$_{h}$, C$_{2}$$_{v}$ and D$_{3}$$_{h}$, can be identified, with the actual point group de- pending on the amplitude of the vibration. In other words, by performing a bend vibration, the molecule is able to intersect the different electronic states associated with these point groups. These intersection points provide passages to lower-lying states. 

These lower states may be dissociative. Therefore, a tendency towards iso- merisation and dissociation may be expected, particularly for small homonuclear clusters. This corroborates the relevance of caging through a rigid shell of atoms, moieties or solvent molecules. In fact, deposition of metal clusters into argon matrices and argon droplets has made the observation of photoluminescence possible (Felix et al., 2001; Sieber et al., 2004; Conus et al., 2006; Harbich et al., 2007). Argon shells have also been used to cage oxygen molecules; the observed luminescence was attributed to oxygen atoms that had first dissociated but were then forced, by the cage, to recombine (Laarmann et al., 2008). 

In summary, non-adiabatic decay at the surface of homonuclear clusters is an important relaxation channel in terms of its competition with fluorescence. The design and engineering of fluorescent clusters therefore requires that this 

