  We present infrared and millimeter observations of molecular gas, dust and
ionized gas towards a sample of clouds distributed along the 500 central pc of
the Galaxy. The clouds were selected to investigate the physical state, in
particular the high gas temperatures, of the Galactic center region (GCr)
clouds located far from far-infrared of thermal radio continuum sources. We
have found that there is ionized gas associated with the molecular gas. The
ionizing radiation is hard (~35000 K) but diluted due to the inhomogeneity of
the medium. We estimate that ~30 % of the warm molecular gas observed in the
GCr clouds is heated by ultra-violet radiation in photo-dissociation regions.
