  High spectral resolution ISO-LWS observations of the [OI] 63.2 and 145.5
microns and [CII] 157.7 microns fine structure lines are presented for the
center of the Sagittarius B2 complex (SgrB2). Both the [OI] 63.2 microns and
the [CII] 157.7 microns lines are detected in absorption. Using observations of
the CO isotopes and of the HI lines, absorption components can be associated
with many clouds along the Sgr B2 line of sight. From these data, we were able
to disentangle three different layers, as predicted by PDR models, which
contain atomic oxygen. We derive lower limits for the column densities of C and
O. The fact that the [CII] 157.7 microns line is detected in absorption implies
that the main cooling line of the interstellar medium can be optically thick
especially in the direction of large star-forming complexes or in the nuclei of
galaxies. This could partially account for the deficiency in the [CII] 157.7
microns line which has been recently found toward infrared bright galaxies in
ISO data.
