Final relative integrated optical depth A_f/A_i of C-H stretching modes, obtained after infinite irradiation as function of the electronic cross section S_e for the different irradiation ion/energy of a-C:H 1 samples. Two different fits corresponding to the linear increase of the recombination volume V with S_e (Eq. (6)) are plotted: the solid line (fit 1) is the best fit, and the dashed line (fit 2) represents an extreme fit for which the asymptotic A_f/A_i are the highest for low S_e. The coloured arrows represent the S_e range of the few major cosmic ray elements.