When crystals of Alkali Halides are exposed to high energy radiation such as x-rays or electrons, point defects are created within the crystal.

These point defects add more energy levels to the atoms in the crystal (similar to impurity energy levels in semiconductors (see 6.3)).
These extra energy levels can cause optical absorption at specific wavelengths, thus adding color to the transparent Alkali Halides. These colors gave the name color center lasers to these lasers.

There are few kinds of defects in crystals, but for our purpose we shall explain the simple defect called F-center (from the German word "Farbe" for color).

Without giving specific details about energy levels in crystals, it is enough to describe the defect in the crystal as causing local region with extra positive charge. This region can be regarded as a "nucleus" around which electrons can assembled similar to Hydrogen atom (see Bohr model of the atom).
The electron is bound to a positive Halogen vacancy.
The excited electron energy states in the lattice are strongly coupled to lattice vibrations. Thus, all electronic states are broadband, resulting in broad absorption and emission bands.