Description
We present observations of the infrared fine-structure lines of [Si II] (34.8{mu}m), [O I] (63.2 and 145.5{mu}m) and [C II] (157.7{mu}m) obtained with the ISO SWS and LWS spectrographs of nine Planetary Nebulae (PNe). These lines originate in the Photo-Dissociation Regions (PDRs) associated with the nebulae and provide useful information on the evolution and excitation conditions of the ejected material in these regions. In order to interpret the observations, the measured line intensities have been compared with those predicted by photo-dissociation models.
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