Examples of the destruction of the aliphatic C-H stretching IR absorption feature around 2900 cm^-1 during ion irradiation of a-C:H 1 a), a-C:H 2 b), and soot c). The upper (red) line is the initial spectrum, before irradiation. The optical depth decreases (from green to purple lines in the online coloured version) as the fluence rises. The fluence between two spectra is of the order of few 10¹² and few 10¹³ ions cm^-2 for the Si⁷⁺ and C⁶⁺ irradiations, respectively.