Spectroscopy Reveals Details of the Hope Diamond
The Naval Research Laboratory in Washington has been synthesizing diamonds since the mid-1980s, hoping to use them as thermal, optical and electrical semiconducting materials for a variety of US Department of Defense applications. Because high quality is essential, the researchers also have been studying the impurities of natural diamonds to determine the significance of the defects seen in synthetic ones.
Since early 2005, James Butler and other scientists from the laboratory’s chemistry division have been collaborating with Jeffrey Post, curator of the natural gem and mineral collection at the Smithsonian Institution, to study a variety of colored diamonds — most notably the Hope diamond, the largest known natural blue diamond. They are especially interested in blue diamonds because of their semiconducting electrical properties.
Scientists have long known that the Hope diamond exhibits a long-lived reddish-brown phosphorescence, which is very rare in natural diamonds. However, although this phosphorescence has been photographed, it had never been subjected to scientific investigation. The researchers decided to conduct spectroscopy tests on the Hope and other diamonds and, to this end, enlisted the help of Roy A. Walters of Ocean Optics Inc. in Winter Park, Fla.
Researchers have begun to characterize the fluorescence and phosphorescence properties of the Hope diamond and other gems. Ultimately, this could help them understand and characterize defects in natural diamonds, which would contribute to the synthesis of diamonds for defense applications.
Together, the researchers explored the optical absorption, Raman spectroscopy, fluorescence and phosphorescence of the diamonds, paying particular attention to the spectral and temporal properties of the phosphorescence. Although little is known about the phosphorescence of natural diamonds, studies have revealed that these properties are the result of ultraviolet-activated defects within the gems. Thus, probing the phosphorescence properties can lead to understanding the defects.
For most of the ultraviolet/visible light studies, they used one of Ocean Optics’ USB spectrometers. The company’s deuterium/quartz halogen light source provided excitation; single fibers and seven-fiber bundles delivered and collected the light. For the Raman studies, they used a 785-nm solid-state laser and a near-infrared spectrometer, also provided by the company.
The investigators considered performing laser-induced breakdown spectroscopy on the Hope diamond. This technique uses a laser to ablate a small amount of material, which then breaks down into excited ionic and atomic species. This briefly provides access to the characteristic atomic emission lines of the elements in the specimen.
“Early on, they wanted to perform [the technique] on the diamond to determine its actual content,” Walters said. “What makes it blue? Of course, it is well-known that boron and nitrogen interstitials cause the blue, but is this always true, and could there be something else?” Before using the technique on the Hope diamond, Walters asked Michael Krzemnicki of the Swiss Gemological Institute in Basel to try it on other diamonds, blue and otherwise. “It left behind a small black smudge,” he said. “Upon viewing this on a visit to Basel, I decided not to pursue this test on such a national treasure as the Hope.”
The researchers are analyzing the data obtained from the Smithsonian collection. Although they have not yet linked phosphorescence to specific defects within the diamonds, they have begun to characterize them based on their fluorescence and phosphorescence properties. The emission wavelengths range from the blue-green to the red, Walters said, usually in broad though somewhat isolated bands: sometimes in one band, other times in two or even perhaps three or four. The peak wavelengths vary from diamond to diamond; even more significantly, the decay times do as well. “In general, each fluorescing and phosphorescing stone has its own signature,” he said.
As for the Hope, Walters said it is evident that the diamond is flawless. Also, when the scientists illuminated it with the cone of light from an optical fiber, only the illuminated region emitted light. “There seems to be no coupling of the color center excitation to adjacent regions,” he explained, “even though diamonds are the best conductors of heat available.”
Finally, the diamond has two distinct broad bands of fluorescence and phosphorescence: one weak at 500 nm and one very strong at 663 nm. The former decays rapidly and the latter more slowly, permitting measurement for as long as one minute. “So it appears to decay with a quick change in color,” Walters said.
- Pertaining to optics and the phenomena of light.
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