  We present Spitzer-IRS spectra obtained along the molecular jet from the
Class 0 source L1448-C (or L1448-mm). Atomic lines from the fundamental
transitions of [FeII], [SiII] and [SI] have been detected showing, for the
first time, the presence of an embedded atomic jet at low excitation. Pure
rotational H$_2$ lines are also detected, and a decrease of the
atomic/molecular emission ratio is observed within 1 arcmin from the driving
source. Additional ground based spectra (UKIRT/UIST) were obtained to further
constrain the H$_2$ excitation along the jet axis and, combined with the 0--0
lines, have been compared with bow-shock models. From the different line
ratios, we find that the atomic gas is characterized by an electron density n_e
~ 200-1000 cm^{-3}, a temperature T_e < 2500 K and an ionization fraction <~
10^{-2}; the excitation conditions of the atomic jet are thus very different
from those found in more evolved Class I and Class II jets. We also infer that
only a fraction (0.05-0.2) of Fe and Si is in gaseous form, indicating that
dust still plays a major role in the depletion of refractory elements. A
comparison with the SiO abundance recently derived in the jet from an analysis
of several SiO sub-mm transitions, shows that the Si/SiO abundance ratio is
~100, and thus that most of the silicon released from grains by sputtering and
grain-grain collisions remains in atomic form. Finally, estimates of the atomic
and molecular mass flux rates have been derived: values of the order of
~10$^{-6}$ and ~10$^{-7}$ M$_{\sun}$ yr$^{-1}$ are inferred from the
[SI]25$\mu$m and H$_2$ line luminosities, respectively. A comparison with the
momentum flux of the CO molecular outflow suggests that the detected atomic jet
has the power to drive the large scale outflow.
