  High-resolution R~50 000 long-slit spectroscopy of the inner knots of the
highly symmetrical protostellar outflow HH 212 was obtained in the 1-0 S(1)
line of H2 at 2.12 micron with a spatial resolution of ~0.45 arcsec. At the
resulting velocity resolution of ~6 km s-1, multiple slit oriented observations
of the northern first knot NK1 clearly show double-peaked line profiles
consistent with either a radiative bow shock or dual (forward and reverse)
shocks. In contrast, the velocity distribution of the southern first knot SK1
remains single-peaked, suggesting a significantly lower jet velocity and
possibly a different density variation in the jet pulses in the southern flow
compared to the northern flow. Comparison with a semi-empirical analytical
model of bow shock emission allows us to constrain parameters such as the bow
inclination to the line of sight, the bow shock and jet velocities for each
flow. Although a few features are not reproduced by this model, it confirms the
presence of several dynamical and kinematical asymmetries between opposite
sides of the HH 212 bipolar jet. The position-velocity diagrams of both knots
exhibit complex dynamics that are broadly consistent with emission from a bow
shock and/or jet shock, which does not exclude jet rotation, although a clear
signature of jet rotation in HH 212 is missing. Alternative interpretations of
the variation of radial velocity across these knots, such as a variation in the
jet orientation, as well as for the velocity asymmetries between the flows, are
also considered. The presence of a correlation between flow velocity and
collimation in each flow is suggested.
