  We present large scale 9' x 27' (25 pc x 70 pc) far-IR observations of the
Sgr B2 complex using the spectrometers on board the Infrared Space Observatory
(ISO). The far-IR spectra are dominated by the strong continuum emission of
dust and by the fine structure lines of high excitation potential ions (NII,
NIII and OIII) and those of neutral or weakly ionized atoms (OI and CII). The
line emission has revealed a very extended component of ionized gas. The study
of the NIII 57 microns/NII 122 microns and OIII 52/88 microns line intensity
ratios show that the ionized gas has a density of n_e~10^{2-3} cm^-3 while the
ionizing radiation can be characterized by a diluted but hard continuum, with
effective temperatures of ~35000 K.
  Photoionization models show that the total number of Lyman photons needed to
explain such an extended component is approximately equal to that of the HII
regions in Sgr B2(N) and (M) condensations. We propose that the inhomogeneous
and clumpy structure of the cloud allows the radiation to reach large distances
through the envelope. Therefore, photodissociation regions (PDRs) can be
numerous at the interface of the ionized and the neutral gas. The analysis of
the OI (63 and 145 microns) and CII (158 microns) lines indicates an incident
far-UV field (G_0, in units of the local interstellar radiation field) of
10^{3-4} and a H density of 10^{3-4} cm^{-3} in such PDRs. We conclude that
extended photoionization and photodissociation are also taking place in Sgr B2
in addition to more established phenomena such as widespread low--velocity
shocks.
