The present work submits the results of an investigation of diamond polycrystalline films, synthetic diamond powders and single crystals. Using photoacoustic spectroscopy, it was found that in the optical absorption spectra of all the three types of diamond materials studied there is a band corresponding to inter band absorption for quanta with energy exceeding 5.5 eV. For single-crystal samples an absorption band due to nitrogen impurities was detected and for polycrystalline films and powders there was observed a broad band due to structural defects. The specific feature of the spectra of films and powders is a strong light scattering in the ultraviolet region (250-300 nm), the efficiency of which increased with decrease of crystallite size from $\sim$100 to $\sim$1 μm. Further decrease of the size of diamond particles (down to 0.25 μm and less) diminished the share of scattered light because of the change of particles faceting and their coalescence.
$^1$Department of Physics of Polymers and Crystals, Faculty of Physics, Moscow State University, Leninskie Gory, Moscow, 119992, Russia
$^2$Tokai University, Kitakaname, Kanagawa, Japan
$^3$Electrochemical Laboratory, Tsukuba, lbaraki, Japan