Phosphorescence
Phosphorescence
Persistent afterglow following ultraviolet excitation
Phosphorescence is a form of luminescence in which a material continues to emit light after the exciting energy source — typically ultraviolet radiation — has been removed. It is distinguished from fluorescence, which ceases essentially immediately when the exciting source is switched off, by the duration of the afterglow. In gemmological practice, phosphorescence is observed under the same long-wave and short-wave ultraviolet lamps used to test for fluorescence, by recording any glow that persists once the lamp is turned off.
Mechanism
Phosphorescence arises from electronic transitions involving metastable energy states. When ultraviolet radiation excites electrons in the gemstone to higher energy levels, most return to the ground state quickly by emitting fluorescent light. A subset, however, become trapped in metastable states from which the spin-forbidden transition back to the ground state proceeds far more slowly, releasing photons over seconds, minutes, or in extreme cases hours. The exact decay time depends on the trap depth, on temperature, and on the specific defects or activators in the material.
Common activators in gemstone phosphorescence include defects involving boron, nitrogen, and various transition-metal impurities, as well as crystallographic defect states unconnected to specific impurity atoms.
Phosphorescence in gem materials
Type IIb diamond — boron-bearing, electrically semiconducting blue diamond — is the most diagnostically important phosphorescent gem material. Type IIb diamonds typically phosphoresce blue to red after short-wave ultraviolet excitation, with a characteristic decay time of seconds to a minute or more. The Hope Diamond is the most famous example, and the phosphorescence of Type IIb diamond is part of the diagnostic toolkit used by GIA and other laboratories to support the identification of natural Type IIb material and to distinguish it from synthetic counterparts, which can also phosphoresce but with different spectral characteristics.
Certain fluorites — particularly some chlorophane-bearing varieties — show prolonged phosphorescence after both ultraviolet and thermal excitation, and the term phosphorescent is sometimes applied loosely to fluorite specimens that glow in the dark for extended periods. Some calcites, willemites, and synthetic strontium aluminate materials show long-decay phosphorescence as well.
Diamond phosphorescence is also observed in some chemical-vapour-deposition synthetic diamonds and some high-pressure-high-temperature synthetic diamonds, and the spectral and decay-time characteristics of synthetic-diamond phosphorescence form part of the laboratory data used in synthetic diamond identification.
Practical observation
For routine gemmological practice, phosphorescence is tested by exposing the stone to short-wave ultraviolet radiation in a darkened viewing cabinet for a period of seconds, then switching off the lamp and observing the stone for any persistent glow. Long-wave excitation can also induce phosphorescence in some materials but is less commonly diagnostic.
The presence, colour, and decay time of phosphorescence are recorded as part of a complete gemmological workup. Misidentification of phosphorescence as fluorescence — and vice versa — is a common informal mistake; a careful test that observes the stone after the lamp is removed, rather than only while it is on, is essential.