  We investigate the diagnostic capabilities of the iron lines for tracing the
physical conditions of the shock-excited gas in jets driven by pre-main
sequence stars. We have analyzed the 300-2500 nm X-shooter spectra of two jets
driven by the pre-main sequence stars ESO-Halpha 574 and Par-Lup 3-4. Both
spectra are very rich in [FeII] lines over the whole spectral range; in
addition, lines from [FeIII] are detected in the ESO-H\alpha 574 spectrum. NLTE
codes along with codes for the ionization equilibrium are used to derive the
gas excitation conditions of electron temperature and density, and fractional
ionization. The iron gas-phase abundance is provided by comparing the iron
lines emissivity with that of [OI] 630 nm. The [FeII] lines indicate ESO-Halpha
574 jet is, on average, colder (T_e = 9000 K), less dense (n_e = 2 10^4 cm^-3)
and more ionized (x_e = 0.7) than the Par-Lup 3-4 jet (T_e = 13000 K, n_e = 6
10^4 cm^-3, x_e < 0.4), even if the existence of a higher density component
(n_e = 2 10^5 cm^-3) is probed by the [FeIII] and [FeII] ultra-violet lines.
Theoretical models suggest that the shock at work in ESO-Halpha 574 is faster
and likely more energetic than the Par-Lup 3-4 shock. This latter feature is
confirmed by the high percentage of gas-phase iron measured in ESO-Halpha 574
(50-60% of its solar abundance in comparison with less than 30% in Par-Lup
3-4), which testifies that the ESO-Halpha 574 shock is powerful enough to
partially destroy the dust present inside the jet. This work demonstrates that
a multiline Fe analysis can be effectively used to probe the excitation and
ionization conditions of the gas in a jet without any assumption on ionic
abundances. The main limitation on the diagnostics resides in the large
uncertainties of the atomic data, which, however, can be overcome through a
statistical approach involving many lines.
