The 3 μm region spectrum, presented as optical depth, for a = 3 nm a-C(:H) particles for increasing band gap, E_g =  −0.1 to 0.25 eV (upper, grey-black), E_g = 0.5 eV (middle, pink), E_g = 0.75 eV (lower, red, y-axis intercept at  ≈ 0.12) and E_g = 1 to 2.67 eV (lower, yellow-purple, bottom to top at  ≈ 3.45 μm). The thin, grey, vertical lines indicate the band origins (see Paper I): aromatic (short), olefinic (medium) and aliphatic (long), CH (solid), CH₂ (dashed) and CH₃ (dashed-triple dotted). The data with error bars (in grey) are for the diffuse ISM line of sight towards the Galactic Centre source IRS6E and Cyg OB2 No. 12 (taken from Pendleton & Allamandola 2002). For comparison the observational data are scaled to the E_g = 2.67 eV data.