Deep-red Alexandrite from Brazil
A small (6mm) crstal of alexandrite (Be Al2 O4) from Carnaiba, Bahia, Brazil obtained from Jaroslav Hyršl. This is coloured by a relatively high concentration of Cr3+ resulting in a very weak green transmission band - not good news for being able to see the celebrated 'colour-change' of this mineral.
The transmission spectrum (red line) shows the spin-forbidden transitions in the red (analogous to the ruby lines) as absorption (Fano resonances). Although seen in a number of chromium coloured crystals such as emerald and kyanite, these resonances are not seen clearly in ruby itself since, for this structure, the spin-allowed and spin-forbidden lines do not have a very significant wavelength overlap.
The image - using both transmitted and reflected light - shows the strong absorption in this crystal. The two fluorescence spectra: 365nm LED (blue line) and 407nm laser (green line) differ due, I think, to the difference of illumination geometry resulting in a stronger self absorption of the 407nm excited fluorescence within the crystal. This means that while the R-lines appear weakly in emission around 680nm in the 365nm spectrum, they are in absorption in the laser-excited signal.
Deep-red Alexandrite from Brazil
A small (6mm) crstal of alexandrite (Be Al2 O4) from Carnaiba, Bahia, Brazil obtained from Jaroslav Hyršl. This is coloured by a relatively high concentration of Cr3+ resulting in a very weak green transmission band - not good news for being able to see the celebrated 'colour-change' of this mineral.
The transmission spectrum (red line) shows the spin-forbidden transitions in the red (analogous to the ruby lines) as absorption (Fano resonances). Although seen in a number of chromium coloured crystals such as emerald and kyanite, these resonances are not seen clearly in ruby itself since, for this structure, the spin-allowed and spin-forbidden lines do not have a very significant wavelength overlap.
The image - using both transmitted and reflected light - shows the strong absorption in this crystal. The two fluorescence spectra: 365nm LED (blue line) and 407nm laser (green line) differ due, I think, to the difference of illumination geometry resulting in a stronger self absorption of the 407nm excited fluorescence within the crystal. This means that while the R-lines appear weakly in emission around 680nm in the 365nm spectrum, they are in absorption in the laser-excited signal.