of the much smaller number of electrons. The IP is of order 15.1 eV. Such large IP’s are typical of covalent systems of small to moderate size. The optical response, as well, is typical of covalent molecules with its highly fragmented structure above the IP, and some isolated low energy peaks below the IP. Associated time scales lie well below fs. Ionic vibrations are more energetic than in other systems because of the especially light H species and the strong covalent binding between H and O. The O-H ionic vibration energy is about 0.5 eV with associated period of 8.3 fs.

All in all, the four above examples point out the diversity and richness of the various systems nowadays accessible to both experimental and theoretical investigations. The various cases also show that the range of energy and time scales to be investigated is rather large from attosecond to several fs for electrons, and from fs to ps for ions. In addition, the optical spectra exhibit different pattern. Specific for metals is the especially well marked Mie surface plasmon with simple scaling properties with size [@Bra93]. The case of covalent systems is more involved with basically no simple scaling properties, but 