  We have observed the [CII] 158 micron line emission from the Galactic plane
(-10 deg < l < 25 deg, |b| <= 3 deg) with the Balloon-borne Infrared Carbon
Explorer (BICE). The observed longitudinal distribution of the [CII] line
emission is clearly different from that of the far-infrared continuum emission;
the Galactic center is not the dominant peak in the [CII] emission. Indeed, the
ratio of the [CII] line emission to far-infrared continuum (I_[CII] / I_FIR) is
systematically low within the central several hundred parsecs of the Galaxy.
The observational results indicate that the abundance of the C+ ions themselves
is low in the Galactic center. We attribute this low abundance mainly to soft
UV radiation with fewer C-ionizing photons. This soft radiation field, together
with the pervasively high molecular gas density, makes the molecular
self-shielding more effective in the Galactic center. The self-shielding
further reduces the abundance of C+ ions, and raises the temperature of
molecular gas at the C+/C/CO transition zone.
