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  Australian scientists explain photochromism phenomenon in pink diamonds


January 12, 2013


A group of scientists at the University of Western Australia have managed to explain the photochromic behavior of pink diamonds, which refers to the stone's tendency to change of color when exposed to light.

Writing in the journal, Diamond and Related Materials, the lead author Keal Byrne explained that natural pink diamonds have defect centers that allow optically controllable and reversible photochromism at room temperature.

"We have pumped these diamonds with various wavelengths of light and measured the response in both time and absorption intensity," he wrote. "What we've seen is that the diamond color - the amount of absorption that gives it the pink color - is dependent on both the wavelength and intensity of the light, and what that is consistent with is a model of electron transfer between the unknown pink defect and other defects in the lattice."

The paper explained that the pink coloration arises from "absorption bands centered at 550 nanometers and 390 nanometers." The diamond can be "bleached," or made lighter, under ultraviolet illumination, while the darker color can be restored with longer-wavelength light.

The researcher wrote that the pumped-in light caused competing types of photoionization at different defect centers, producing photochromism. They found that while the defect centers are responsible for other diamond colors, among them blue and yellow, the defects themselves do not explain how pink diamonds get their specific hue. The color center responsible for the pink coloration, however, is still unknown.

"So we're trying to work out what these new energy levels are that are unique to the pink diamond and by that way we can work out what properties it has and what use it might have," Byrne wrote.

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