Top: Herschel/HIFI spectra towards W3-IRS5. The black line shows the [C ii] line at 1.9 THz, the red line shows the [C i] line at 492 GHz, and the blue line the ground state transition of HF at 1.2 THz. The horizontal axis is the LSR velocity in km s^-1 and the vertical axis the antenna temperature in Kelvins. The continuum level for [C ii] corresponds to the SSB continuum level. Bottom: zoom on the [C i] and HF lines (red and blue as above), and average [C ii] spectrum of the OFF positions (black). The continuum levels have been shifted for clarity in the bottom panel.