Canonical interstellar H₂ formation rate constant (grey horizontal band, Gry et al. 2002) compared to that predicted for H₂ formation via CH bond UV photo-dissociation from a-C:H/a-C grains, k_f[CH]/n_H = 2.0 × 10^-17 cm³ s^-1 shown at a = 0.5 nm (squares with error bars). The green green square also includes H₂ formation in the a-C mantles on the large a-Sil_Fe grains into the calculation, yielding k_f[CH]/n_H = 4.0 × 10^-17 cm³ s^-1. The small squares show the relative contributions to the total H₂ formation rate as a function of particle radius. The H atom sticking rate constant, for s_H = 1, is shown for the small a-C (thin grey), large a-C:H/a-C (thick grey) and a-Sil/a-C (green) grain distributions and for gas temperatures of 50, 100 and 200 K (short-dashed, solid and long-dashed, respectively).