Photochromism of H2 and H3 centres in synthetic type Ib diamonds

Photochromism has been studied for irradiated type Ib diamonds which are then annealed above 1500 degrees C. This mainly involves H2 and H3 centres. When the samples are illuminated with light of wavelength shorter than 600 nm, absorption of H2 centres is reduced while that of H3 centres is enhanced. In the dark the process is reversed. Several characteristic features have been examined for various illumination conditions and different samples. On the basis of the results a model is proposed: H2 and H3 centres are the same centre with different charge states and photoionization of H2 may be responsible for the photochromism.