The cover picture shows a section of the polyhedral representation of the structure of potassium zeolite L (K9L) (SiO4, AlO4 tetrahedra, potassium atoms as spheres, the colors of which distinguish the three different crystallographic positions). When K9-L is exposed to a defined amount of potassium vapor, neutron diffraction shows that potassium atoms are incorporated only at the partially occupied K(4) sites (green) that are arranged about the large one-dimensional channels of the zeolite framework. These atoms are ionized, and the electrons released remain primarily within these channels, to compensate for the excess positive charge on the K(4) positions. Investigations with ESR spectroscopy showed that if one electron is confined to a single channel, it would be delocalized over six to eight unit cells (a distance of 4.5 to 6.0 nm). P. A. Anderson, A. R. Armstrong, and P. P. Edwards report more on the controlled introduction of electrons in microporous solids on p. 641 ff.