  Magnetic precursors of C-shocks accelerate, compress and heat molecular ions,
modifying the kinematics and the physical conditions of the ion fluid with
respect to the neutral one. Electron densities are also expected to be
significantly enhanced in shock precursors. In this Letter, we present
observations of strongly polar ion and neutral molecules such as SiO, H13CO+,
HN13C and H13CN, which reveal the electron density enhancements associated with
the precursor of the young L1448-mm outflow. While in the ambient gas the
excitation of the ions and neutrals is explained by collisional excitation by
H2 with a single density of 10E5cmE-3, H13CO+ shows an over excitation in the
shock precursor component that requires H2 densities of a factor of >10 larger
than those derived from the neutral species. This over excitation in H13CO+ can
be explained if we consider an additional excitation by collisions with
electrons and an electron density enhancement in the precursor stage by a
factor of 500, i.e. a fractional ionization of 5x10E-5. These results show that
multiline observations can be used to study the evolution of the ion and
electron fluids at the first stages of the C-shock interaction.
