  We report near-infrared spectroscopic data for the Seyfert 2 galaxy Mrk 78,
taken with the LIRIS near-infrared camera/spectrometer at the William Herschel
Telescope (WHT). The long-slit spectra clearly show extended emission. The
resolution and depth of the near-infrared spectra allows the examination of its
morphology and ionization regions, and a direct comparison with similarly deep
visible spectra. The emission-line ratios obtained are used to derive the
extinction towards the nucleus. The detection of strong features such as [Fe
II], H$_{2}$, hydrogen recombination lines and the coronal [Si
VI]$\lambda$1.962 line is used to study the kinematics and excitation
mechanisms occurring in Mrk 78, revealing that despite of the strong radio-jet
interaction present in this object, photoionization from the active nucleus
dominates the narrow line region emission, while UV fluorescence is the source
of the H$_{2}$ emission. Lines with extended emission yield velocity
distributions with an amplitude of about 600 km/s, the consequence of an
eastern lobe moving away from us plus a western lobe with the opposite
contribution. We used the photoionization code CLOUDY to recreate a typical
narrow line region region, to derive the ionization parameter, and to compare
our spectral data with diagnostic diagrams.
