Derived imaginary part, k, of the nano-diamond complex indices of refraction as a function of radius (0.5, 1, 3, 10, 30, and 100 nm: thick red, orange, yellow, green, blue, and violet, respectively). The thin lines show the same radii optEC_(s)(a) data for aliphatic-rich (E_g = 2.67 eV) particles (size colour-coding as for nano-diamonds). The thin black line shows the bulk diamond data (Edwards & Philipp 1985) and the short-dashed and long-dashed black line shows the data for pre-solar nano-diamonds of Mutschke et al. (2004) and Lewis et al. (1989), respectively. We note the 100 nm 7− 30 μm data (violet line) delineates the input neutron-irradiated, N-poor bulk diamond absorbance data. The thin grey vertical lines indicate the diamond two and three phonon mode peak positions, the thin blue vertical lines mark the hydrogenated nano-diamond 3.43 and 3.53 μm bands and the thin yellow vertical lines approximately indicate the characteristic type Ib diamond IR band positions and also highlight the ≃22 μm neutron-irradiated N-poor diamond band. The fractional surface hydrogen coverage, f_H = 1, 0.25 and 0, decreases from the upperto lower panels, respectively. The data in left panels assumes ‘pristine’ bulk diamond and that in the right panels neutron-irradiated bulk diamond (Hill et al. 1998). A zoom into the mid-IR region of these data is shown in Fig. 3.