Electron density and carriers of the diffuse interstellar bands

We have used the ionisation equilibrium equation to derive the electron density in interstellar clouds in the direction of 13 stars. A linear relation was found that allows determination of the electron density from the Mg I and MgII column densities in diffuse clouds. The comparison of a normalised equivalent width of 12 DIBs with the electron density shows that the DIBs equivalent width do not change with electron density varying in the range ne = 0.01 divided by 2.5 cm(-3). Therefore the DIBs carriers (1) can be observed only in one ionisation stage, or (2) the DIBs are arising in cloud regions (eg. cores or cloud coronas) for which we cannot determine the electron density.