We present ISO-SWS and ISOPHOT-S spectroscopy Based on observations made with ISO , an ESA project with instruments funded by ESA member states ( especially the PI countries : France , Germany , The Netherlands and the United Kingdom ) and with the participation of ISAS and NASA . of the Seyfert galaxy NGC 4151 .
We detect a total of 17 fine-structure emission lines emitted by a wide range of low- and high-excitation ions , two rotational lines of molecular hydrogen , and the Br \beta HI line .

We find that the mid-IR fine-structure line profiles display blue asymmetries which are very similar to those observed in the optical lines produced in the narrow line region .
Because the mid-infrared lines are much less sensitive to extinction than are the optical lines this similarity places strong constraints on scenarios which have been invoked to explain the optical line asymmetries .
For example , we are able to rule out the simplest radial-motion-plus-dust scenarios for the production of the line asymmetries .
Our preferred model is that of a central , geometrically thin but optically thick , obscuring screen of sub-arcsecond extension , enclosing a total hydrogen gas mass of \gtrsim 5 \times 10 ^ { 6 } M { \odot } .
This mass may be molecular .

The weakness of ‘ PAH ’ emission features in the low resolution spectrum is evidence that star formation plays a minor role in the circumnuclear region of NGC 4151 .
In a companion paper , we use the rich set of mid-infrared lines to determine the obscured photoionizing continuum produced by the active galactic nucleus .

\keywords galaxies : individual : NGC 4151 – galaxies : Seyfert – line : profiles – infrared : galaxies