In a methane/argon discharge used for deposition of hydrogenated amorphous carbon (a‐C@B:H), energetic ion bombardment yields radicals on solid surfaces by two mechanisms: (i)fragmentation of hydrocarbon ions at their impact on the surface and (ii)sputtering of the already depositeda‐C:H film. To discriminate between these two mechanisms, the emission intensity of CH(A‐X) in the vicinity of a negatively biased electrode was measured as a function of the ion impact energy. The threshold energy for yielding the excited CH radical was found to be ∼2 eV for fragmentation and ∼80 eV for sputtering. The fragmentation yield is much larger on a metal surface than ona‐C:H layer. The sputtering yield dominates over the fragmentation yield for the high impact energy(>150 eV).