  We present deep spectroscopic (3600 - 24700 A) X-shooter observations of the
bright Herbig-Haro object HH1, one of the best laboratories to study the
chemical and physical modifications caused by protostellar shocks on the natal
cloud. We observe atomic fine structure lines, HI, and He, recombination lines
and H_2, ro-vibrational lines (more than 500 detections in total). Line
emission was analyzed by means of Non Local Thermal Equilibiurm codes to derive
the electron temperature and density, and, for the first time, we are able to
accurately probe different physical regimes behind a dissociative shock. We
find a temperature stratification in the range 4000 - 80000 K, and a
significant correlation between temperature and ionization energy. Two density
regimes are identified for the ionized gas, a more tenuous, spatially broad
component (density about 10^3 cm^-3), and a more compact component (density >
10^5 cm^-3) likely associated with the hottest gas. A further neutral component
is also evidenced, having temperature lass than 10000 K and density > 10^4
cm^-3. The gas fractional ionization was estimated solving the ionization
equilibrium equations of atoms detected in different ionization stages. We find
that neutral and fully ionized regions co-exist inside the shock. Also,
indications in favor of at least partially dissociative shock as the main
mechanism for molecular excitation are derived. Chemical abundances are
estimated for the majority of the detected species. On average, abundances of
non-refractory/refractory elements are lower than solar of about 0.15/0.5 dex.
This testifies the presence of dust inside the medium, with a depletion factor
of Iron of about 40%.
