The Cause Of Photoluminescence

(via Wahroongai News, Volume 30, No.5, May 1996) Grahame Brown writes:

Many minerals (gemstones) owe their photoluminescent (fluorescent and phosphorescent) properties to the presence of fluorescence activators in their crystal structure. These activators may be either elemental ionic impurities, metallic radicles, e.g., urinate, or structural defects in the crystal structure. Fluorescence activators function by introducing required properly separated raised energy levels for electrons in the mineral’s crystal structure. It is the presence of these defined energy levels (above the ground state) that will allow the mineral to luminesce when irradiated by short wavelength visible light, ultraviolet and x-ray wavelengths, and cathode rays (accelerated electrons).

Substitution of activator ions into a crystal structure is controlled by both the size (radius) and charge of the substituting ions. If a size or charge mismatch does occur, then smaller size and higher charge will affect ionic substitution in crystal structure.

The energy levels required for luminescence may be either:
- Associated with either a single activator, e.g., the europium ions responsible for the blue fluorescence of fluorite.
- Shared with a second co-activator, e.g., the copper and aluminum impurities in sphalerite that are responsible for its fluorescence.
- Shared between the host mineral and an activator, e.g., the green fluorescence of willemite is the result of shared energy levels between the willemite silicate structure and its manganese activator.

In some minerals, e.g., diamond, non-metals such as nitrogen and hydrogen act as fluorescence activators. While there is no lower limit on the concentration of an activator required to produce fluorescence, typical fluorescence activators are present in quite small concentrations that may vary from 1ppm to several percent. However, higher concentrations of activators can inhibit fluorescence by the process known as concentration quenching. A good example of concentration quenching is provided when the concentration of Cr³+ ions start to absorb their neighbors fluorescent emission, thus eventually quenching the red fluorescence of the ruby. Other causes of fluorescence quenching include heat and the additional presence of ions that poison luminescence.

Certainly fluorescence can be quenched by heat. This occurs because heat increases molecular vibrations in the crystal structure. As a consequence, these additional vibrations can interact with an activator or co-activator (sensitizer) to carry off their luminescence excitation energy as heat.

Fluorescence also can be quenched if certain impurities occur in a mineral that has the potential for photoluminescence. For example, the presence of Fe³+ and to a lesser extent Fe²+ in ruby effectively quenches its fluorescence; for the Fe³+ selectively steals blue to ultraviolet wavelengths from Cr³+ to energize a charge transfer reaction between Fe³+ and its surrounding oxygen atoms. Consequently the Cr³+ ion can’t be energized, and minimal red fluorescence (purely from red and green absorption) will occur.

For more information refer to Robbins, M (1994); Geoscience Press, Phoenix.