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Ionized carbon is the main gas-phase reservoir of carbon in the neutraldiffuse interstellar medium and its 158 micron fine structure transition [CII]is the most important cooling line of the diffuse interstellar medium (ISM). Wecombine [CII] absorption and emission spectroscopy to gain an improvedunderstanding of physical conditions in the different phases of the ISM. Wepresent high resolution [CII] spectra obtained with the Herschel/HIFIinstrument towards bright dust continuum sources regions in the Galactic plane,probing simultaneously the diffuse gas along the line of sight and thebackground high-mass star forming regions. These data are complemented byobservations of the 492 and 809 GHz fine structure lines of atomic carbon andby medium spectral resolution spectral maps of the fine structure lines ofatomic oxygen at 63 and 145 microns with Herschel/PACS. We show that thepresence of foreground absorption may completely cancel the emission from thebackground source in medium spectral resolution data and that high spectralresolution spectra are needed to interpret the [CII] and [OI] emission and the[CII]/FIR ratio. This phenomenon may explain part of the [CII]/FIR deficit seenin external luminous infrared galaxies. The C+ and C excitation in the diffusegas is consistent with a median pressure of 5900 Kcm-3 for a mean TK ~100 K.The knowledge of the gas density allows us to determine the filling factor ofthe absorbing gas along the selected lines of sight: the median value is 2.4 %,in good agreement with the CNM properties. The mean excitation temperature isused to derive the average cooling due to C+ in the Galactic plane : 9.5 x10^{-26} erg/s/H. Along the observed lines of sight, the gas phase carbonabundance does not exhibit a strong gradient as a function of Galacto-centricradius and has a weighted average of C/H = 1.5 +/- 0.4 x 10^{-4}.

[CII] absorption and emission in the diffuse interstellar medium across the Galactic Plane