The results of optical absorption analysis for the synthetic diamond (type Ib) modified by the molecular hydrogen ion with 40 keV, 6.3 10 1 6 H/cm 2 , at room temperature, showed that the increase of optical density of the modified layer (∼140 nm) in the UV-VIS region resulted from the point defects generated by ion collisions. The position-sensitive spectrophutometer is useful for the characterization of optical inhomogeneity of the type Ib diamond and heavy damaged surface layer on a homojunction diamond substrate.