  We report on the results of infrared spectroscopic mapping observations
carried out in the nuclear region of Centaurus A (NGC5128). The 500 pc bipolar
dust shell discovered by Quillen et al.(2006) is even more clearly seen in the
11.3 micron dust emission feature than previous broad band imaging. The pure
rotational lines of molecular hydrogen other than the S(0) line are detected
above the dusty disk and associated with the oval dust shell. The molecular
hydrogen transitions indicate the presence of warm gas at temperatures
250--720K. The ratio of the dust emission features at 7.7 and 11.3 micron and
the ratio of the [NeII](12.8) and 11.3 dust emission feature are lower in the
500 pc dust shell than in the star forming disk. The clearer shell morphology
at 11.3 micron, warm molecular hydrogen emission in the shell, and variation in
line ratios in the shell compared to those in the disk, confirm
spectroscopically that this shell is a separate coherent entity and is unlikely
to be a chance superposition of dust filaments. The physical conditions in the
shell are most similar to Galactic supernova remnants where blast waves
encounter molecular clouds. The lines requiring the highest level of
ionization, [NeV] and [OIV], are detected 20--25 arcsec north-east and
south-west of the nucleus and at position angles near the radio jet axis. Fine
structure line ratios and limits from this region suggest that the medium is
low density and illuminated by a hard radiation field at low ionization
parameter. These higher S molecular hydrogen pure rotational transitions are
also particularly bright in the same region as the [OIV] and [NeV] emission.
This suggests that the gas associated with the dust shell has been excited near
the jet axis and is part of an ionization cone.
