Aside from the N3-related emission, which is not active with excitation energy above 435 nm, sample 7 showed a 520 nm emission band with a maximum at 470 nm excitation. This feature is caused by the H3 center, which was observed from photoluminescence spectra in these three samples but not in the other four from this category. As stated earlier, most diamonds contain multiple color centers.
H3 and H4 Center. The H3 center consists of a vacancy trapped at an A aggregate of nitrogen, which gives an uncharged defect consisting of two nitrogen atoms separated by a vacancy—for instance, (N-V-N)0. Optical absorption at this center has a ZPL at 503 nm and creates a yellow bodycolor in diamond, while also commonly producing green luminescence with visible-light stimulation. The H4 center consists of four nitrogen atoms separated by two vacancies. It is created when a vacancy migrates through the diamond lattice and combines with a B aggregate of nitrogen. This center has a ZPL at 496 nm and typically produces yellow color in diamond. Both H3 and H4 centers can occur naturally or during treatment (Collins, 1982).