Over the past two years, the heat treatment of corundum involving lattice diffusion of beryllium (Be) at temperatures over 1800°C has become a major issue in the gem trade. Although initially only orange to orangy pink ("padparadscha"-like) sapphires were seen, it is now known that a full range of corundum colors, including yellow and blue as well as ruby, have been produced or altered by this treatment. An extension of the current understanding of the causes of color in corundum is presented to help explain the color modifications induced by Be diffusion. Empirical support is provided by Bediffusion experiments conducted on corundum from various geographic sources. Examination of hundreds of rough and faceted Be-diffused sapphires revealed that standard gemological testing will identify many of these treated corundums, although in some instances costly chemical analysis by mass spectrometry is required. Potential new methods are being investigated to provide additional identification aids, as major laboratories develop special nomenclature for describing this treatment.
Shells and pearls often show iridescent color. The cause of this phenomenon has been attributed to diffraction, both diffraction and interference, or interference alone. We used a shell of the mollusk Pinctada margaritifera, which shows very strong iridescent colors, to study how this color is produced in the layers of nacre in shells. From observations with a scanning electronic microscope (SEM), this particular shell exhibits a very fine scale diffraction grating structure. This suggests that the iridescent color is caused by diffraction, which was demonstrated by an experiment using an argon ion laser illuminating the shell to produce a distinct diffraction image. The strength of the iridescent color can be correlated to both the groove density of the diffraction grating formed by the shell, and the surface quality of the grooves themselves. A shell with a high groove density and a smooth groove surface produces a strong iridescent color.
Quantum-beat spectroscopy has been used to observe excited states of the N-V center in diamond. For the 1.945-eV optical transition, direct evidence is presented for the existence of GHz-scale fine structure, together with a much larger 46-cm Ϫ1 level splitting in the E state. An interference effect observed in transient fourwave-mixing response is explained with a polarization selection rule involving Zeeman coherence among magnetic sublevels. Also, detailed dephasing measurements versus temperature and wavelength have identified the decay mechanisms operative among the various states. A comparison of these results with ab initio calculations of excited electronic structure and interactions based on several multielectron models supports the conclusion that the N-V center is a neutral, two-electron center governed by a strong Jahn-Teller effect and weak spin-spin interactions.
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